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A Polyextreme Hydrothermal System Controlled by Iron: The Case of Dallol at the Afar Triangle
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ACS Earth and Space Chemistry

Cite this: ACS Earth Space Chem. 2019, 3, 1, 90–99
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https://doi.org/10.1021/acsearthspacechem.8b00141
Published December 6, 2018

Copyright © 2018 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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One of the latest volcanic features of the Erta Ale range at the Afar Triangle (NE Ethiopia) has created a polyextreme hydrothermal system located at the Danakil depression on top of a protovolcano known as the dome of Dallol. The interaction of the underlying basaltic magma with the evaporitic salts of the Danakil depression has generated a unique, high-temperature (108 °C), hypersaline (NaCl supersaturated), hyperacidic (pH values from 0.1 to −1.7), oxygen-free hydrothermal site containing up to 150 g/L of iron. We find that the colorful brine pools and mineral patterns of Dallol derive from the slow oxygen diffusion and progressive oxidation of the dissolved ferrous iron, the iron-chlorine/-sulfate complexation, and the evaporation. These inorganic processes induce the precipitation of nanoscale jarosite-group minerals and iron(III)-oxyhydroxides over a vast deposition of halite displaying complex architectures. Our results suggest that life, if present under such conditions, does not play a dominant role in the geochemical cycling and mineral precipitation at Dallol as opposed to other hydrothermal sites. Dallol, a hydrothermal system controlled by iron, is a present-day laboratory for studying the precipitation and progressive oxidation of iron minerals, relevant for geochemical processes occurring at early Earth and Martian environments.

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Introduction

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The dome of Dallol lies in the Danakil depression at the extension of the Main Ethiopian Rift (MER) of the Afar Triangle (Figure 1a). This depression, situated 120 m below sea level (mbsl), is a vast salty plain composed of more than 2 km of evaporites created from successive marine transgressions of the Red Sea, the most recent placed at 32 kya, during the evolution of the MER since Miocene to Quaternary. (1) Categorized as BWh (hot desert climate) according to the Köppen classification (2) with scarce annual precipitation (up to 144 mm but usually below 50 mm) and a mean annual temperature of 35 °C, (2) the area of Danakil is one of the driest and hottest places on the planet. Danakil belongs to the dry drainage basin of Dinakle with no surficial flow and no, or insignificant, flow out of the drainage system. (3) However, the karstified Jurassic limestones and sandstones that underlay the evaporitic sequence behave as aquifers enabling the infiltration of meteoritic water from the Danakil Alps to the east and the highlands of the North Ethiopian plateau to the west, via the normal faulting and transverse fractures of the rift zone. In this region of incipient seafloor spreading, where the crustal thickness does not exceed 14 km, the Red Sea, the Gulf of Aden, and the MER produce a triple junction zone associated with mantle plume activity and basaltic intrusive and extrusive magmatism. (4−7) As a result, several volcanic chains are forming, mainly represented by basaltic shield volcanoes, among them the NNW-SSE Erta Ale range. The Dallol dome along with the neighboring areas of the Black and Gaet’ Ale Lakes (the latter also known as Yellow Lake) (8) are the most recent expressions at the northwestern part of the Erta Ale volcanic range; following from SE to NW the Hayli Gubbi, Ale Bagu, Erta Ale, Borale, Dalafilla, Alu, and Gada Ale volcanoes (9,10) (Figure 1a,b). The precise age of the Dallol dome is unknown, probably counting a few hundreds of years, whereas Black and Yellow lakes formed after the phreatic explosions of 1926 and 2005, respectively. Upwelling of basaltic magma confined in a magma chamber just 2.4 km deep below the segment of Dallol intruded the marine evaporitic sequence and created a dome structure lying 60 mbsl. (9,11,12) Dallol has not yet developed into a typical volcano; the ascending of magmatic fluids rich in CO2, SO2, H2S and acidic gases, together with the boiling of meteoric water and of the seawater trapped in the evaporitic sequence, led to the generation of a hydrothermal system of hyperacidic springs with high content of dissolved CO2 and chlorides. (9,11−13) This system is also characterized by an impressive palette of colors that is related to the high iron concentration of the fluids, deriving from the dissolution of the underlying basalt and iron-rich formations (14) (Figures 1 and 2). In this work we unravel the geochemical processes of this singular geological site.

Figure 1

Figure 1. The hydrothermal system of Dallol within the Erta Ale range and maps of the active hydrothermal features of January 2017. (a) Dallol dome located in Danakil depression within the Erta Ale volcanic range of the Afar Triangle. (b) Geological map of the wider area of the Dallol dome, including Black and Yellow Lakes. (c) Aerial image of Dallol dome, showing the active areas in January 2017. (d) Map of January 2017 of the hydrothermal activity showing the referred sampling sites within the text, spring (S1), gray spring (S3) and a system of four successive pools (P1–P4).

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Figure 2

Figure 2. Representative image of the hydrothermal system of Dallol. Fumaroles and active hydrothermal chimneys forming terraces and pools are shown at the background of the image. Moving downward from the terraces, the oxidation of the Fe mineral precipitates is manifested by the change in the color from white to green, yellow, and finally to red. Pillars formed by exhausted springs of past hydrothermal activity are shown at the upper left and right of the image. The height of the right pillar is 1.5 m.

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Experimental Section

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In Situ Measurements

Temperature (±0.1 °C) and pH (±0.1) were measured, both for spring and pools, with a Hanna pH meter equipped with a glass electrode. Prior to the pH measurement the electrode was soaked in sulfuric acid (97.5%) for 24 h, whereas for the pH calibration we used buffers of pH 4, 2, 1.68, and 0.8. The pH meter was calibrated prior to every measurement and each measurement was repeated three times and in different days. Also, dissolved oxygen (μmol/L) was measured with the high sensitivity PreSens Fiber Optic Oxygen Meter Fibox 4, of temperature, automatic pressure and salinity compensation, and detection limit of 15 ppb dissolved oxygen, 0–100% oxygen. The PreSens system was calibrated with a two-point calibration before each measurement. Temperature, pH, and oxygen concentration measurements were repeated three times for the same sampling sites in different days during the fieldtrip.

Sampling and Preservation of the Anoxic Conditions

For the preservation of the pristine conditions of the liquids and to avoid the oxidation during the sample transportation and the measurements, samples of spring and pool water were collected in 12 mL sterile glass vials sealed with a septum tap, covered with parafilm tape and were kept inside a N2 atm glovebox. Selected efflorescences and precipitates were preserved also in sterile glass septum sealed vials and in resealable with ziplock seal bags and were kept inside the glovebox. Bubbles of free gas were trapped within a plastic bucket with a pierceable tap. The gas was collected with a syringe and transferred to a 12 mL vial (Exetainer, Labco Limited, United Kingdom) previously filled with brine from the same sampling point. The gas displaces and replaces the brine until only a tiny residual liquid remains in the vial. Samples were collected in triplicate. For the analysis of dissolved gases, samples were stored in 12 mL vials with a pierceable tap (Exetainer, Labco Limited, United Kingdom). The vials were filled as much as possible, avoiding bubbles. About 1 mg of mercury chloride was added to avoid biological activity.

Chemical Analysis of Water

Because of the high salinity of the brines, each sample within the 12 mL vials were diluted up to 50 mL and then they were filtered by 0.45 μm. For the determination of the concentration of the anion species we used a 940 Professional IC Vario instrument (Metrohm, Herisau, Switzerland) equipped with a conductivity detector at the Centro de Instrumentación Científica (CIC) of the University of Granada, Spain. An isocratic gradient of Na2CO3 (3.6 mM) was used as eluent, keeping an eluent flow at 0.7 mL min–1. The injection loop was 50 μL. Analysis was done in an ionic resin column Metrosep A Supp 7–250/4.0. For the rest of the major, minor, and trace elements, samples were analyzed using an axial inductively coupled plasma optical emission spectrometer (axial ICP-OES Agilent (Varian) 720-ES). ICP analyses were carried out at the Instrumental Technical Services of the Estación Experimental del Zaidı́n (CSIC). Analytical quality control included analysis of one duplicate, three blank solutions, as well as analysis of a series of appropriate reference materials.

Raman and UV–vis Spectroscopy

Raman analysis of the pristine spring and pool water, preserved in oxygen-free conditions, was performed in the laboratory of the Espectrocopı́a Raman e Infrarroja aplicado a Cosmogeoquı́mica y Astrobiologı́a (ERICA) of Unidad Asociada UVA-CSIC, Spain. Spectra were recorded with a Horiba-JY Induram spectrometer and a Laser Elforlight G4–30 PSU (532 nm) Cabezal Raman Horiba-JY Superhead, with a Zeiss 20× LF objetive. The laser potential was set at 20 mW and the water samples were placed vertically with respect to the beam. No sample preparation was required and the samples were not opened during the measurement.
For the investigation of the Fe aqueous species, we used the UNICAM UV300 spectrometer of the Inorganic Chemistry Department of the University of the Granada, Spain. Sample preparation was carried out inside a N2 atm glovebox to preserve the oxygen-free conditions. Because of the very high amount of Fe in the liquids, a one-step dilution of 50 μL of sample into 1 mL of oxygen-free Milli-Q water was done prior to the measurement and samples were placed into quartz cells with septum seal. Spectra were recorded in absorbance mode, between 200 and 600 nm wavelengths, with a resolution of 1.5 nm. The (FZ) method used to determine Fe speciation in the liquids is detailed in the Supporting Information Methods 1.2.

Isotopic Analysis

Isotopic measurements were carried out in the Stable Isotope Laboratory of the Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR, Granada, Spain). For the analysis of δD and δ18O in water, an aliquot of water (0.7 μL) was injected onto a ceramic column containing a glassy carbon tube at 1450 °C to produce H2 and CO gases. (15) A high-temperature reactor (TC/EA) coupled online via a ConFlo III interface to a Delta XP isotope ratio mass spectrometer (Thermo-Finnigan, Bremen). These gases were separated by chromatography using a helium carrier gas stream. To avoid memory effects, each sample was analyzed 10 times by discarding the first 5 analyses and averaging the last five. Commercial CO and H2 bottles and five different waters, previously calibrated versus V-SMOW, SLAP, and GIPS, were used as internal standards for the oxygen and hydrogen isotopic analyses. The precision was calculated higher than 0.2 permil for the oxygen and 1 permil for the hydrogen, whereas the standard for reporting oxygen and hydrogen is V-SMOW (Vienna Standard Mean Ocean Water). The isotopic composition of nitrogen, carbon, and sulfur was analyzed by means of a Carlo Elba NC1500 (Milan, Italy) elemental analyzer with a Delta Plus XL (ThermoQuest, Bremen, Germany) mass spectrometer (EA-IRMS). Sulfur samples were mixed with vanadium pentoxide to ensure complete combustion. Internal standard previously calibrated vs IAEA-S1, IAEA-S2, IAEA-S3, NBS-127 y CP-1 for sulfur and NBS-22, IAEA-CH-7, IAEA-CH-6 for carbon were used. The methodology for the dissolved inorganic carbon (DIC) analysis is described in the Supporting Information Methods 1.2.

Dissolved Gases

For dissolved gas, a head space of 4 mL was created with ultrapure helium (99.9999) in the laboratory. The gases were analyzed in a continuous flow system that includes a liquid nitrogen trap, which prevents water vapor and CO2 passing through the silica capilar, so that only a mixture of He (carrier), N2, O2 and Ar enters the spectrometer. The ratios O2/Ar, N2/Ar, CO2/Ar, O2/N2 and isotopic composition of 15N/14N,18O/16O, 17O/16O, 36Ar/40Ar,38Ar/40Ar were analyzed with a mass spectrometer (Delta V plus; ThermoFinnigan, Bremen, Germany) that was specifically configured with 11 faraday cups to simultaneously analyze N2, O2, CO2, and Ar. This allowed the maximum precision in the mixture of atmospheric gases. The precision was calculated for four bottles of internal standards versus air and was better than ±0.03‰ for δ15N, ±0.05 δ17O, ±0.02 δ18O, ±0.06 δ36Ar, ±0.1 δ38Ar, ±0.8 for δO2/N2 (‰), ±0.9 for δO2/Ar (‰), ±0.56 for δCO2/Ar (‰). The methodology used for the in situ incubations is described in Supporting Information Methods 1.2.

High-Resolution Powder X-ray Diffraction (HRPXRD)

For the detection of the crystalline phases of the mineral precipitates we used a high-resolution Bruker D8 Advance Series II X-ray diffractometer using monochromatic Cu Kα1 radiation with a primary Ge(111) monochromator and a PSD Lynxeye detector at the facilities of the Laboratorio de Estudios Crystalográficos (LEC) of the Instituto Andaluz de Ciencias de la Tierra (IACT) in Granada, Spain. Samples were run in transmission mode, at 40 kV acceleration voltage, and at 40 mA current for 16 h from 4° to 90° 2θ, 0.014° per second. Phase identification was carried out with the Diffrac.Suite software and the American Mineralogist Crystal Structure Database.

Field Emission Gun Scanning Electron Microscopy (FESEM-EDS)

A ZEISS SUPRA 40 VP, FESEM-EDS equipped with an Oxford energy-dispersive X-ray spectrometer (EDS) of the Centro de Instrumentación Cientı́fica (CIC) of the University of Granada, Spain, operating at 5–20 keV was used for the textural and chemical characterization of the submicron mineral phases. High-resolution images were obtained at 5 kV and at 2 mm working distance, in SE and 15 kV in BS mode, whereas for the elemental analyses operating conditions were set at 15 kV accelerating voltage and 7.5 mm working distance and the AZtec 3.0 SP1 EDS software was used. Biological sample preparation for the SEM study is specified in the Supporting Information Methods 1.3.

Micro-Raman

The micro-Raman study of the solids was performed in the laboratory of the Espectroscopı́a Raman e Infrarroja aplicado a Cosmogeoquı́mica y Astrobiologı́a (ERICA) of the Unidad Asociada UVA-CSIC, Spain. The spectra were obtained with a Kaiser OSI HoloSpec f/1.8i. CCD Andor DV420A-OE-130 spectrometer equipped with a Research Electro-Optics LSRP-3501, He–Ne 632.8 nm laser, and a Cabezal Raman Kaiser OSI MKII, HFPH-FC-S-632.8 with a Nikon Eclipse E600 microscope with Nikon 50× by 100× LF objectives. The spot size was 38 μm for the 50× objective and 15 μm for the 100× objective.

Transmission Electron Microscopy

The study in the nanoscale was performed with a Titan X-FEG G2-S/TEM, image Cs-corrector, operating at a 300 kV acceleration voltage at the Centro de Instrumentación Científica (CIC) of the University of Granada, Spain. Samples were placed on a Formvar C coated Cu-grid. Z-contrast images were collected using a high-angle annular dark-field (HADDF) detector in scanning transmission mode (STEM), whereas the chemical composition of the studied phases was obtained with EDX. Also, high-resolution transmission electron microscopy (HRTEM), along with fast fourier transform images (FFT), and selected area electron diffraction (SAED) were performed to investigate the crystal structure of the studied nanophases.

Results and Discussion

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A Highly Active, Polyextreme Hydrothermal System

The crater of the Dallol dome hosts numerous springs, extended fumarolic fields, subaerial and subaqueous hydrothermal springs, and acidic brines that produce salt chimneys, pillars, terraces, and pools of different ephemeral colors (Figure 2). The hydrothermal system is highly dynamic; active spring sites go inactive and new springs emerge in new locations in the range of days, while the activity is changing in the range of hours within the same site. Eleven spring sites occupying an area of 0.038 km2 were active in 2016, whereas 18 spring sites covering a 0.144 km2 area where active in 2017, along NNE trending fissures (see Figure 1B and Supporting Information Figure S-1). However, the dominant geochemical characteristics of the system remained within a narrow range of values between the two field campaigns. The springs are discharging oxygen-free, hyperacidic, Fe-rich hydrothermal brines, which are supersaturated with respect to halite as soon as they are in contact with the atmosphere. The mean concentrations of the major elements of the spring brines are Cl (251.4 g/L) and Na (92.2 g/L) followed by Fe (22.5 g/L), K (10 g/L), S (3.6 g/L, measured as sulfate), Mg (3.7 g/L), and Ca (2.7 g/L), whereas the most abundant trace elements are Mn (714 ppm), F (302 ppm), Br (288 ppm), Al (279 ppm), and Zn (54 ppm) (see Supporting Information Table S-1). The composition of the gases emitted from the fumarolic fields and the spring complexes of Dallol is dominated by CO2 (ranging from 70 to 99%) along with H2S, N2, and traces of H2, Ar, and O2 (Table S-2). (13)
The temperature of the spring water varies between 105.6 (±0.1) °C and 108.4 (±0.1) °C. Upon discharge, the water from the brines evaporates and the pressure and temperature decrease abruptly, triggering almost instantaneous precipitation of halite, creating pillars up to few meters in height. As the water flows downstream from the springs, a series of self-organized halite terraces pool the brines, which reduces turbulent flow. The temperature of the upper pools, those closest to the spring, is 85–71 (±0.1) °C, whereas it drops down to 32 (±0.1) °C due to increasing equilibration time with the atmosphere and evaporative cooling. The pH values of the spring water are close to zero ranging from 0.16 to −0.5 (±0.1). These hyperacidic values decrease from the upper to the lower pools, the lowest pH value measured in both field missions was −1.7 (±0.1) (Table S-1). Hyperacidic values have been reported as well for several volcanic crater lakes and hot springs, among them the Kawah Ijen crater lake in Indonesia (pH ∼ 0.3), (16) the Poas crater in Costa Rica (pH ∼ −0.89), (17,18) the Ebeco Volcano in Kuril Islands (pH ∼ 0.2) (19) the Copahue in Argentina (pH ∼ 0.2), (20−22) the Ruapehu in New Zealand (pH ∼ 0.6), (23) the Nevado del Ruiz, Colombia (pH ∼ 1), (24) and the Santa Ana Volcano, El Salvador (pH ∼ 0.5). (25) Concerning acid mine effluents, hyper-acidic to acidic pH values (0.9–3) have been measured in Rio Tinto in Spain, (26) while (27) reporting a bizarre much lower value of pH, −3.6, from the Richmond Mine of the Iron Mt in California. Thus, to the best of our knowledge, the hydrothermal system of Dallol is the most acidic natural system.
Figure 3 shows the main acidic components of Dallol (sulfates, chlorides, fluorides, and dissolved iron) in comparison with those of other hyperacidic volcano-hydrothermal systems. Dallol fluids exhibit positive anomalies in chlorides (>200 g/L), derived from the interaction of ascending magmatic fluids rich in acidic gases (e.g., HCl(g), HF(g)) with the marine evaporites (containing halite, sylvite, carnallite, gypsum, anhydrite) and in dissolved iron originated from the interaction of the fluids with the underlain basalts and Fe-rich formations. The sulfate values (∼5200 ppm) are lower than the mean values of other volcano-hydrothermal systems, whereas the mean fluoride concentration (∼252 ppm) is comparable to the concentration of the other systems. Consequently, the hyper-acidic pH of the Dallol spring water (pH ∼ 0) results from the presence of strong acids and the formation of Fe(II)-chloro-complexes/-sulfates that are favored in these conditions of which the gradual oxidation and concentration in the hydrothermal pools decreases further the pH down to negative values.

Figure 3

Figure 3. Comparative bar graph of sulfate, chloride, fluoride, and iron concentrations of Dallol (purple-shaded area) and other hyperacidic volcanic sites and the Red Sea brines. The values for Copahue, (22) Poas, (17,18) Ruapehu, (23) Nevado del Ruiz, (24) Ebeko, (19) Santa Ana, (25) Kawah Ijen, (16) Red Sea brines, (28) and Dallol (this study) acidic fluids are shown in ppm.

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Unlike the Dallol hydrothermal brines, neighboring Black Lake is a magnesium chloride pond of 2.5 (±0.1) pH and 60 °C (±0.1) temperature, whereas Yellow Lake is mainly composed of calcium chloride along with magnesium and potassium chlorides of 1.7 (±0.1) pH and 38 °C (±0.1) temperature (Figure 1b and Table S-1). The surrounding area of the salt plain, composed of Na/Ca-K/Mg/Fe-salts and -sulfates, and clays (29) had a pH of 3.5 (±0.1) when flooded with perennial water.

The Color Palette of Dallol

One of the most striking features of Dallol is the broad palette of colors of the brines and the solids, controlled by the iron redox chemistry. Because of the near zero pH of the spring water and the low dissociation constants of hydrogen sulfide, (30,31) the concentration of sulfide in the fluid is practically zero. Sulfur is mainly found around the fumarolic sites as millimeter-to centimeter dull yellow spheroids made of colloidal and poorly crystalline sulfur formed through atmospheric hydrogen sulfide oxidation. On the other hand, sulfates are present in the brines due to the dissolution of the sulfate salts of the evaporitic sequence. Therefore, the aqueous chemistry and the color palette of Dallol should be dominated by the formation of different iron species in both reduced and oxidized states. (32) To determine the redox state across the aqueous system of Dallol, we quantified the concentration of Fe(II) and Fe(III) in different spring solutions and related pools. Fe(II) concentration was determined by employing the external chelator ferrozine and by using UV–vis spectroscopy, Fe(Tot) was measured by ICP-MS/OES, and Fe(III) was obtained as the difference (Figure 4 and Supporting Information Methods 1.1. and Figure S-2). The measured values show that Fe(Tot) is practically ferrous in the spring water and is gradually oxidized to the pools until is completely converted to ferric iron in the lowest most concentrated pools (Figure 4 and Figure S-2). This gradual oxidation of the Fe(II) is reflected in both the pH and the color of the pools. The pH decreases from −0.79 (±0.1) in pool 1 to −1.32 (±0.1) in pool 4 followed by a change in the color of the pools from light green to dark brown due to hydrolysis and concentrative evaporation of the Fe(III) species that release further H+. However, the heat produced by the exothermic Fe(II) oxidation and the increase in the thermal energy by light absorption in the dark brown color pools are responsible for the increase in temperature from 32 (±0.1) °C in pool 3 to 40 (±0.1) °C in pool 4. The high consumption rate of oxygen due to oxidation of the Fe(II) and the precipitation of the Fe(III) minerals explains the anoxic conditions in the lowest pools of the hydrothermal field (Figure 4, pool 4).

Figure 4

Figure 4. Fe(II)/Fe(III) ratio, pH, O2 concentration, and temperature of four successive pools of a typical spring-terrace system. Pool 1 (P1), the upper pool (oxygen-free spring is at the left of the pool, out of the field of view), is dominated by Fe(II) species of light green color. Moving downstream from the spring to pool 3 (P3), temperature is decreasing and the atmospheric O2 diffusion results to the formation of Fe(III) species and to pH drop. Ultimately, the lowest pool 4 (P4) is characterized by Fe(III), the precipitation of which darkens the color of the pool, lowers further the pH, increases the temperature, and consumes the oxygen.

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We used Raman and UV–vis spectroscopies to identify the Fe aqueous species responsible for the colors of the pools. The light green color of the spring and upper pools is due to ferrous species, mainly associated with the v1(A1) (Fe–OH2) stretching band at 370 cm–1 of the [FeII(H2O)6]2+ and the broad band around 420–480 cm–1, although, in this region there is an overlap with the deformation modes of the sulfate/bisulfate anions (Figure 5a, spring and pool 1). (33,34) Moving from the upper to the lower pools, the [FeII(H2O)6]2+ is oxidized and the hexaquairon complex [FeIII(H2O)6]3+ undergoes hydrolysis to form progressive hydroxo-species, namely [Fe(OH)(H2O)5]2+ and [Fe(OH)2(H2O)4]+ with the accompanying pH drop (eqs 1 and 2)
(1)
(2)

Figure 5

Figure 5. Raman and UV–vis spectra of the spring water (S1) and the four gradual pools (P1–P4) of Figure 1 and Figure 4. (a) Region of the Raman spectra showing the main vibrations of the Fe aqueous complexes. The concentration of the Fe(III) is increasing gradually from pool 2 to pool 4, as shown by the Fe(III)-Cl stretching vibration band at 312 cm–1. This correlates with the decrease of the intensity of the Fe(II)–OH2 stretching vibration at 370 cm–1. (b) UV-absorbance spectra showing the increase of the band centered at 330 nm from the spring and upper pool to the lowest pools, due to Fe(II) oxidation and Cl-complexation.

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Furthermore, the very low pH and the high chloride concentration of the brines favor the formation of a complex mixture of Fe(III)-chlorides, such as [FeCl2(H2O)5]2+ and [FeCl2(H2O)4]+ (32,35,36) The latter is the most abundant and was identified by the 312 cm–1 band arising from the v (Fe3+-Cl) stretching vibration in the spectra of the middle and lower pools (37) (Figure 5a). Note that the intensity of the band at 312 cm–1 is increasing from pool 2 to pool 4 as the concentration of Fe(III) is increasing, whereas the 370 cm–1 band decreases dramatically as the Fe(II) species decrease (Figure 5a). Likewise, the intensity of the v1(A1) vibration of the free SO42– at 982 cm–1 is decreasing from the spring water to pool 4 and equilibrates with the v(HO-S) and the SO3 vibrations of the bisulfate anion at 875 and at 1052 cm–1, respectively (Figure 5a, pools 3 and 4). (33,34) The UV–vis spectra of springs and pools consist of a broad absorption band centered around 330 nm that becomes more intense as we move from the spring to pool 4 (Figure 5b).
The major electronic transitions for the Fe(III) hydroxo- and chloro-complexes are attributed to ligand-to-metal charge transfers (LMCT). The LMCT absorption of the hydroxo-complexes occurs around 300 nm, whereas in the chloride-complexes this absorption intensifies and shifts to lower energies up to 360 nm. (36) Therefore, and in accordance with the Raman study, the UV–vis spectra show that the Fe(III)-chlorides are practically absent in the spring and the upper pools and become predominant in the lower pools, which explains the intense yellow color. Finally, the lowest pools show darker color due to further oxidation that provokes the precipitation of hydrated yellow iron-oxyhydroxides/-oxides/-sulfates. Two main mineral phases have been identified by X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy and energy-dispersive analysis, transmission electron microscopy, and diffraction: jarosite (KFe3+3(SO4)2(OH)6) and akaganeite (β-Fe3+O(OH,Cl)), products of the Fe(III) hydrolysis in the presence of sulfate and chloride, respectively.

Mineral Patterns

Regarding the solids halite is the dominant macrocrystalline phase found all over the Dallol dome, producing an impressive array of complex mineral patterns (Figure 6). Among them we underline the following: (i) hydrothermal salt-pillars up to 4 m high, (ii) water-lily structures forming in subaqueous springs, (iii) flower-like crystals growing in lower pools by extreme evaporation, (iv) egg-shaped thin crusts, hollow twisted tubes, and pearl-like spheres around active gas gateways, (v) various types of efflorescences, and (vi) polygonal cracking patterns resembling pieces in a chocolate bar (Figure 6). Salt pillars and water lilies form by subaerial and subaqueous halite crystallization, respectively, due to the cooling of the supersaturated brines. Evaporation and evaporative cooling drive the crystallization of the aforementioned structures except for the “chocolate bar” polygonal cracks, probably formed due to cooling stress, water loss, and solidification of a highly viscous sodium chloride, Fe-rich, solution. On top of the halite crystals, microcrystals of Na-/K-/Ca-/Mg-salts, such as, gypsum (CaSO4·2H2O), sodium-sulfates, sylvite (KCl), and carnallite (KMgCl3· 6H2O) are precipitating. Nevertheless, the minor phases of submicrometer Fe-(oxy)hydroxides/sulfates, mainly jarosite (KFe3+3(SO4)2(OH)6) and akaganeite (β-Fe3+O(OH,Cl) that form a colorful veil over all the mineral patterns, are those responsible for the color diversity and color evolution of the solids with time (Figure 6h–m). Far from the active springs and their corresponding pools, the predominant phases are the Fe-oxides mainly represented by hematite (Fe2O3) (Figure 6i–m). Raman scattering from the precipitates did not pick up any organic compounds related to biologic activity, such as bacterial pigments (e.g., carotenoids, chlorophyll) that would produce a set of bands in the region 1000–1600 cm–1 and would affect their color. (38,39) Therefore, the color of the solids starts as white around the springs due to the rapid halite crystallization with fluid inclusions but turns to yellowish and then to orange and brown as the Fe-oxidation, evaporation, and precipitation of jarosite, akaganeite, hematite, and other Fe-oxides takes place as aforementioned. Following aggregation of the Fe-particles, the color intensity is increased to a dark-red, brown-purple color. In addition to the sulfur spheroids mentioned above, few nanoparticles of S° and Fe-sulfides were also found dispersed in halite crystals probably deriving from the reduction of Fe(III) trapped in the halite crystals by the hydrogen sulfide gas.

Figure 6

Figure 6. Pictures, microscopic images, and mineral composition of different Dallol patterns. (a) Water-lily structures created by subaqueous hydrothermal activity. (b) Halite flowers with variable degree of oxidation. (c) Halite with egg-like shapes forming around gas vents. (d) Twisted hollow tubes of halite. (e) Halite pearls. (f) Efflorescences. (g) Chocolate bar cracks formed during crystallization of an Fe-rich brine or melt. (h) FESEM and Fe, K, S, Cl EDX map of a jarosite-rich efflorescence. (i) FESEM image of jarosite spherules. (j) FESEM image of part of a jarosite spherule decorated by akaganeite particles. (k) TEM/HAADF image of the akaganeite nanoparticles. (l) hematite spindles. (m) Micro-Raman spectrum of the hematite spindles. Mineral abbr.: Hl=Halite, Jrs=Jarosite, Na-Jrs=Natrojarosite, Ak=Akaganeite, Gt=Goethite, Fe-OH=Fe-oxyhydroxides, Fe-O=Fe-oxides, Hem=Hematite, Syl=Sylvite, Gp=Gypsum, Anh=Anhydrite, Car=Carnallite, S=Sulfur, Fe-/Mg-/Ca-Sil=Fe-/Mg-/Ca-silicates.

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In contrast to other hydrothermal systems, such as Yellowstone, where the color is clearly associated with microbial biofilms, (40) the entire color palette of Dallol is the result of inorganic processes. This gradual color variation, ranging from pale green to dark brown and reds, is beautifully displayed in Dallol due to the combined action of the continuous discharge of oxygen-free Fe(II)-rich spring brines, the low solubility of oxygen in high temperature, hyperacidic, and hypersaline brines, and therefore the slow oxidation of the Fe(II) species. The prevalence of laminar flow in the terrace-pool system and the dominance of diffusion over convection along the entire aqueous pathway from the springs to the pools slow down the dynamics of the system, enabling the discrete display of the stunning colors.

Isotopic Study

Dallol spring water has δD values similar to the local meteoric water (−21‰ to −8 ‰ versus V-SMOW), (41,42) while the δ18O values of the spring water are shifted to more positive values (+5‰ to +8 ‰ versus V-SMOW) indicative of the meteoric water interaction with the underlying evaporites and basaltic flows (Figure 7) (Table S-3). The isotopic values of the pool water are both δ18O and δD enriched as a result of the extreme evaporation rates of the pool water (δ18O ∼ 10.7‰ and δD > +3.2 ‰). Note that the sample most affected by evaporation corresponds to the lowest, most acidic pool (BP1). Moreover, oxygen isotopic values distinguish Dallol brines from the neighboring sites of Yellow and Black lakes, which exhibit relatively negative δ18O values (−1‰ to −7 ‰), uncommon for the low latitude of the site (41) (Figure 7). This indicates different hydrological processes between the geothermal lakes and the system of the dome, that is also reflected in the difference in the hydrochemistry (see Introduction). As shown from other hyper-saline systems, (43,44) the negative δD values of Black and Yellow lakes can be a result of the very high salinity of these Mg–Ca–Cl-rich lakes that are under severe evaporation processes and exhibit complex evolution.

Figure 7

Figure 7. Stable isotope data for the Dallol hydrothermal brines and Black and Yellow Lake waters (2016 and 2017 campaigns). Dallol spring water has δD‰ values similar to the local meteoric water. The anomalous high δ18O values of the hot springs indicate the interaction of the meteoric water with the underlying basaltic flows and evaporites. The δ18O‰ and δD values of the Dallol pools are higher due to evaporation processes (δ18O > +10 ‰, δD > −10 ‰). Yellow and Black Lake waters exhibit relatively negative values for both δD (−25‰ to −48‰) and δ18O (−1‰ to −7‰) showing distinct hydrological processes with respect to the Dallol dome. MWL, Meteoric Water Line; (41) AAMWL, Addis Ababa Meteoric Water Line. (42) Magmatic and primary magmatic waters are plotted after refs (48−50.)

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Concerning the dissolved gases of the hydrothermal brines of Dallol, the ratio N2/Ar shows an excess of N2 for the spring water indicative of the magmatic influence of the shallow source below the dome, whereas the slightly positive values measured at the pool water result from the subsequent water–atmosphere equilibration (see Supporting Information Table S-4). (45,46) For the free gases Darrah et al., (13) reported more positive δ15N values (+2.93 to +4.5) that were interpreted as a source of meta-sediment decomposition. This isotopic evolution of the free gases toward more negative values (see Table S-2) can be a result of the increased hydrothermal activity over the past years, as shown also from the 2016 and 2017 activity maps of the dome (Figure S-1).
The amount of organic matter in the dome is practically zero except for one spring system of gray color explored during the 2017 mission at the SW of the dome (Figure 1d). The gray color of that spring is owed to amorphous silica and degraded, partially graphitized, organic matter. The silica particles were identified by FESEM as remnants of partially dissolved diatom shells, whereas the measured δ13C values of the organic matter (−13‰) are typical of marine algae. (47) Therefore, both silica and organic matter are considered remnants of fossil diatoms trapped in the evaporitic sequence. For the detection of autotrophic and heterotrophic metabolic activity in the pools, we performed in situ incubations of isotopically labeled 18O enriched water and urea-C, (13) respectively. Both incubations did not identify traces of biological activity (see Supporting Information, Figure S-3 and Table S-5). Likewise, no biologic material was detected during FESEM observation of fixed filters of Dallol brines and no organic compounds were picked up by Raman analysis of numerous efflorescences and precipitates (see Supporting Information Methods for sample preparation and fixation).

Summary and Conclusions

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We report on a terrestrial hydrothermal system that discharges hyperacidic (near to zero pH and down to −1.7), hypersaline and oxygen-free brines that contain up to 150 g/L of iron. We demonstrate that the hyperacidic pH, the brine evolution, the color palette, and the mineral paragenesis are controlled by inorganic processes, related to iron oxidation and iron complexation with chlorides and sulfates. Our results suggest that life, if present, is not involved in the iron cycling and iron mineral precipitation at Dallol, as opposed to other submarine and terrestrial hydrothermal systems. Although it is well-known that life can tolerate or even thrive under extreme conditions, (51) the impact of multiple physicochemical extremes on microorganisms is poorly understood due to the scarcity of such polyextreme environments. (52) If microorganisms are present in Dallol polyextreme hydrothermal brines, their existence would expand the limit of life supporting habitance on Earth and on Earth-like planets, as we currently understand it, rendering Dallol a site of unique astrobiological significance.
At the same time, Dallol is a natural laboratory for the study of the precipitation of iron minerals and one of the very few present-day sites where we can witness the progressive oxidation of the iron formations as possibly occurred in early Earth, during the transition from anoxic-suboxic to oxygen-rich conditions. (53−56) Finally, the mineralogical prevalence of jarosite and related sulfates, and akaganeite and related Fe-oxyhydroxides in the precipitates of Dallol that have been also identified on different Martian geological sites, (57−59) the minor microbiologic and anthropogenic contamination of the dome, as well as, the intense iron corrosion, render Dallol a terrestrial Martian analogue-site of interest for future space missions.

Supporting Information

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TThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.8b00141.

  • 1. Supporting Methods. 1.1. The ferrozine (FZ) method for the Fe speciation in the liquids. 1.2. Isotopic analysis. 1.3. Biological sample preparation for the SEM study. 2. Supporting Results. 2.1. In situ incubations of urea-13C and laboratory experiments. 3. Supporting Figures S-1 to S-3. 3.1. Figure S-1. Aerial photographs and related hydrothermal activity maps of Dallol dome for the 2016 and 2017 field campaigns. 3.2. Figure S-2. Fe species concentration for the spring water (S1) that is dominated by Fe(II) and four successive pools of Dallol (P1–P4) where Fe(III) is gradually increasing over Fe(II). 3.3. Figure S-3. δ13CDIC ‰ of isotopically labeled urea for incubations performed in situ and in the laboratory. 4. Supporting Tables S-1 to S-5. 4.1. Table S-1. Comparative chemical analysis of Dallol brines for the 2016 and 2017 field campaigns 4.2. Table S-2. Oxygen, nitrogen, argon, and carbon stable isotopes and atomic ratios in free gases from springs of Dallol and Black and Yellow Lake waters (sampling 2017). 4.3. Table S-3. Stable isotopes in H2O and DIC (dissolved inorganic carbon) for Dallol hydrothermal brines compared to water from the wider area. 4.4. Table S-4. Oxygen, nitrogen, argon, and carbon stable isotopes and atomic ratios in dissolved gases from Dallol brines and Black and Yellow Lake waters. 4.5. Table S-5. First and second batch of measurements of δ13CDIC‰ of isotopically labeled Urea for incubations performed in selected samples of Dallol pools and sterilized solutions of FeCl2, FeCl3, and (NH4)2S2O8 (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Electra Kotopoulou - Instituto Andaluz de Ciencias de la Tierra, (IACT), 18100 Granada, Spain;  Orcidhttp://orcid.org/0000-0002-5789-846X
    • Antonio Delgado Huertas - Instituto Andaluz de Ciencias de la Tierra, (IACT), 18100 Granada, Spain
    • Jose M. Dominguez-Vera - Departamento de Química Inorganica- Instituto de Biotecnologia, Universidad de Granada (UGR), 18071 Granada, Spain
    • Jose Maria Lopez-Garcia - Instituto Geológico y Minero de España (IGME), 07006 Palma de Mallorca, Spain;  Orcidhttp://orcid.org/0000-0002-8564-9989
    • Isabel Guerra-Tschuschke - Centro de Instrumentación Científica, Universidad de Granada (UGR), 18071 Granada, Spain
    • Fernando Rull - Unidad Asociada UVa-CSIC al Centro de Astrobiología, University of Valladolid, 47002 Valladolid, Spain
  • Author Contributions

    E.K., A.D.H., J.M.L.G., and J.M.G.R. conducted the fieldwork and the sampling of the 2016 and 2017 missions. A.D.H. performed and analyzed the isotopic studies. J.M.D.V. designed and provided guidance for the UV–vis and Fe speciation study of the liquids. J.M.L.G. performed the geological mapping. F.R. provided guidance for the Raman study. I.G.T. conducted the scanning electron microscopy study. E.K. wrote the paper, performed the experiments, and analytical measurements and analyzed the data. J.M.G.R. designed the work and wrote the paper. All authors discussed and contributed to the last version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We acknowledge Olivier Grunewald and IRIS Foundation for organizing and supporting the field trips to Dallol and for supplying terrestrial and aerial photographs. We thank Dr. Tschaye Asmelash and Dr. Makonen Tafari from the University of Mekele, Abdul Ahmed Aliyu from Turism Expansion and Park, and Luigi Cantamessa for logistic help during field trips. We acknowledge Dr. Cristobal Verdugo Escamilla, for technical assistance with the X-ray diffraction, Aurelio Sanz Arranz for assistance with the Raman study, and Arsenio Granados Torres for assistance with the isotopic study. We thank Prof. Purificacion Lopez-Garcia for sharing field and laboratory data and for helpful discussions. This work received funding from the European Research Council under the Programme (FP7/2007-2013)/ERC Grant Agreement 340863 (Prometheus) and from MINECO, ref CGL2016-78971-P, AEI/FEDER, UE”.

References

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    A general-purpose, online, continuous flow method for detn. of δD and δ18O values of water and hydrous minerals is described. Minor modifications of com. available equipment allow for analyses of water and solid samples, fluid inclusions and in situ hydrogen isotope detns. of hydrous minerals using a laser. The technique involves redn. of H2O or solid hydrous samples by reaction with glassy carbon at high temps. H2 and CO are produced by reaction with the carbon at 1450°C in a helium carrier gas. Product gases are sepd. in a gas chromatograph and analyzed in a mass spectrometer configured to make hydrogen isotope analyses in continuous flow mode. Sample size is as small as 0.1 μl of water (or equiv. from hydrous phases) for both hydrogen and oxygen isotope ratio detns. Waters are injected through a heated septa into the He stream; solid materials are wrapped in silver foil and dropped into the furnace using an autosampler. Using std. correction procedures, results obtained with this method are identical to those obtained conventionally with a precision for water samples of ±2‰ (1σ) for hydrogen and ±0.2‰ (1σ) for oxygen. Saline waters can be analyzed without any addnl. prepn. Reproducibility of δD values from hydrous silicates is also ±2‰. The δ18O values of 'dehydration' water evolved from biotite during heating is variable and irreproducible. Total time of anal. is less than 2 min for a single hydrogen isotope anal. Sample size can be reduced an order of magnitude by using a low He-flow rate, a narrow-bore redn. column, a capillary GC column and a more efficient open split. With the high sensitivity design, in situ measurements can be made on hydrous minerals using a CO2 laser for sample heating. Stable isotope detns. of fluid inclusions can be made by decrepitating samples in the He-stream.
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  16. 16
    Delmelle, P.; Bernard, A. Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia. J. Volcanol. Geotherm. Res. 2000, 97, 55– 75,  DOI: 10.1016/S0377-0273(99)00159-6
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    16
    Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia
    Delmelle, P.; Bernard, A.
    Journal of Volcanology and Geothermal Research (2000), 97 (1-4), 55-75CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier Science B.V.)
    The crater lake of Kawah Ijen volcano contains extremely low pH (<0.4) waters with high SO4 (∼70000 mg/kg), Cl (∼21000 mg/kg), F (∼1500 mg/kg), Al (∼5000 mg/kg), Fe (∼2000 mg/kg) and trace metal (Cu ∼0.5, Zn ∼4, Pb ∼3 mg/kg) contents. These brines seep outward through the western crater rim and reappear on the other side as streamlets, which form the headwaters of the Banyupahit stream. The Banyupahit first mixes with fresh rivers and thermal springs in the Ijen caldera and then irrigates a coastal agricultural plain which is 30 km from the summit crater; downstream compn. changes affecting the Banyupahit waters are investigated using stable isotope, chem. and mineralogical data collected from sites along the stream length. The satn. of the stream waters with respect to minerals was evaluated with SOLVEQ and WATEQ4F and compared with the geochem. observations. An aluminous mineralogy (alunogen, pickeringite, tamarugite and kalinite) develops in the upper part of the Banyupahit due to concn. of the headwaters by evapn. Downstream attenuation of dissolved element concns. results principally from diln. and from mineral pptn. The stream pH changes from ∼0 at the source to °4 close to the mouth. The δD and δ18O values and the relative SO4-Cl-F contents of the Banyupahit waters indicate that the tributaries are mostly meteoric. Dissolved SO4 in the acidic stream come only from the crater lake seepages and are not involved later in microbially mediated reactions, as shown by their δ34S and δ18O values. Re-equilibration of the stream SO4 oxygen-isotope compn. with H2O from tributaries does not occur.Calcium, SiO2, Al, Fe, K and SO4 behave non-conservatively in the stream waters. Gypsum, silica (amorphous or poorly ordered), a basic aluminum hydroxysulfate (basaluminite), K-jarosite and amorphous ferric hydroxide may exert a soly. control on these elements along the entire stream length, or in certain stream sections, consistent with the thermochem. model results. Downstream concn. trends and mineral satn. levels suggest that pptn. of Sr-, Pb-rich barite and celestite consume Ba, Sr and Pb, whereas dissolved Cu, Pb and Zn may adsorb onto solid particles, esp. after the junctions of the acidic stream with non-acidic rivers. Significant fluxes of SO4, F, Cl, Al, SiO2, Ti, Mn and Cu may reach the irrigation system, possibly causing serious environmental impacts such as soil acidification and induration.
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  17. 17
    Rowe, G. L.; Brantley, S. L.; Fernandez, M.; Fernandez, J. F.; Borgia, A.; Barquero, J. Fluid-volcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica. J. Volcanol. Geotherm. Res. 1992, 49, 23– 51,  DOI: 10.1016/0377-0273(92)90003-V
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    17
    Fluid-volcano interaction in an active stratovolcano: The crater lake system of Poas volcano, Costa Rica
    Rowe, Gary L., Jr.; Brantley, Susan L.; Fernandez, Mario; Fernandez, Jose F.; Borgia, Andrea; Barquero, Jorge
    Journal of Volcanology and Geothermal Research (1992), 49 (1-2), 23-51CODEN: JVGRDQ; ISSN:0377-0273.
    Seismic and geochem. data collected at Poas volcano, Costa Rica, since 1978 suggest that temp. and chem. variations recorded in subaerial fumaroles and the crater lake are related to episodic release of heat and volatiles assocd. with hydrofracturing of the upper margin of the shallow magma body. Power outputs assocd. with these events approach 600 MW and are superimposed on a baseline energy flux of approx. 200 MW. The baseline heat flux suggests a magma solidification rate of 0.012 km3/yr and background volatile release rates of 1000 H2O, 66, S, 13 Cl, and 0.5 tons/day F. These fluxes are comparable to fluxes of F, Cl, and S exiting the magmatic/hydrothermal system through acidic flank springs and are about a factor of five less than estd. volatile fluxes through summit fumaroles during the high-temp. event of 1981-1983. Mass balance considerations suggest that heat released at the lake surface is primarily supplied by the ascent of heated brines supplemented by condensation of fumarolic steam. Calcd. av. seepage rates out of the lake (≈450 kg/s) indicate rapid convection of acidic lake brine through underlying lake sediments and pyroclastic deposits. Circulation of the extremely corrosive lake brine through the volcanoclastic material beneath the lake may enhance subsurface permeabilities. Fluxes of rock-forming elements obsd. in the Rio Agrio basin on the northwest flank of Poas suggest that dissoln. and removal of volcano-clastic material occurs at a rate of approx. 1650 m3/yr. However, estd. sulfur fluxes from the cooling magma body suggest that porosity created by this dissoln. could quickly be filled by the deposition of native sulfur. Finally, anal. of heat and water budgets for the crater lake over the period 1978-1989 indicates that the recent decline and disappearance of the lake was caused by variations in summit rainfall combined with an increase in subsurface heat flow to the lake in 1987-1988. The increase in heat flow is related to continued degassing of high-temp. volatiles following a hydrofracturing/magma ascent episode that occurred in 1986.
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  18. 18
    Rodríguez, A.; van Bergen, M. J. Superficial alteration mineralogy in active volcanic systems: An example of Poás volcano, Costa Rica. J. Volcanol. Geotherm. Res. 2017, 346, 54– 80,  DOI: 10.1016/j.jvolgeores.2017.04.006
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    18
    Superficial alteration mineralogy in active volcanic systems: An example of Poa´s volcano, Costa Rica
    Rodriguez, Alejandro; van Bergen, Manfred J.
    Journal of Volcanology and Geothermal Research (2017), 346 (), 54-80CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier B.V.)
    The alteration mineralogy in the crater area of Poa´s volcano (Costa Rica) has been studied to constrain acid fluid-rock interaction processes and conditions relevant for the formation of sulfate-bearing mineral assemblages found on the surface of Mars. Individual sub-environments, which include the hyperacid lake (Laguna Caliente), ephemeral hot springs, fumarole vents and areas affected by acid rain and/or spray from the lake, are marked by distinct secondary mineral assocns., with sulfates commonly as prevailing component. The sulfates occur in a wide mineralogical diversity comprising gypsum/anhydrite, various polyhydrated Al-sulfates, alunite-jarosite group minerals, halotrichite-, voltaite- and copiapite-group minerals, epsomite and romerite. Depending on the sub-environment, they are variably assocd. with clay minerals (kaolinite-group and smectite-group), zeolites, SiO2-polymorphs, Fe-(hydro)oxides, Ti-oxides, native sulfur, sulfides, chlorides, fluorides, phosphates and carbonates. Geochem. modeling was performed to identify mechanisms responsible for the formation of the secondary minerals found in the field, and to predict their possible stability under conditions not seen at the surface. The results indicate that the appearance of amorphous silica, hematite, anhydrite/gypsum, pyrite, anatase and kaolinite is relatively insensitive to the degree of acidity of the local aq. system. On the other hand, alunite-jarosite group minerals, elemental sulfur and Al(OH)SO4 only form under acidic conditions (pH < 4). The presence of polyhydrated Mg- and Fe2 +-sulfates is restricted to olivine-bearing rocks exposed to acid rain or brine spray. Modeling suggests that their formation required a repetitive sequence of olivine dissoln. and evapn. in an open system involving limited amts. of fluid. The mineral variety in the crater of Poa´s is remarkably similar to sulfate-bearing assemblages considered to be the product of acid-sulfate alteration on Mars. The analogy suggests that comparable fluid-rock interaction controls operated in Martian volcanic environments.
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  19. 19
    Kalacheva, E.; Taran, Y.; Kotenko, T.; Hattori, K.; Kotenko, L.; Solis-Pichardo, G. Volcano–hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur. J. Volcanol. Geotherm. Res. 2016, 310, 118– 131,  DOI: 10.1016/j.jvolgeores.2015.11.006
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    19
    Volcano-hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur
    Kalacheva, Elena; Taran, Yuri; Kotenko, Tatiana; Hattori, Keiko; Kotenko, Leonid; Solis-Pichardo, Gabriela
    Journal of Volcanology and Geothermal Research (2016), 310 (), 118-131CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier B.V.)
    Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ∼ 1000 m above sea level (asl). The fumaroles are characterized by low-temp., HCl- and S-rich gas and numerous hyper-acid pools (pH < 1) without drains. At ∼ 550 m asl, in the Yurieva stream canyon, many hot (up to 87 °C) springs discharge ultra-acidic (pH 1-2) SO4-Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000-2000 t/d of water vapor, < 20 t/d of SO2 and < 5 t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60-80 t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano-hydrothermal system. Oxygen and hydrogen isotopic compn. of water and Cl concn. for Yurieva springs show an excellent pos. correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evapn. of the acidic brine. Temps. calcd. from gas compns. and isotope data are similar, ranging from 150 to 250 °C, which is consistent with the presence of a liq. aquifer below the Ebeko fumarolic fields. Satn. indexes of non-silicate minerals suggest temps. ranging from 150 to 200 °C for Yurieva springs. Trace elements (including REE) and Sr isotope compn. suggest congruent dissoln. of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compns., suggesting complicated boiling-condensation-mixing processes.
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    Rodríguez, A.; Varekamp, J. C.; van Bergen, M. J.; Kading, T. J.; Oonk, P. B. H.; Gammons, C. H.; Gilmore, M. In Copahue Volcano; Tassi, F.; Vaselli, O.; Caselli, A., Eds.; Springer-Verlag: Berlin, Heidelberg, 2015; pp 141– 172 DOI: 10.1007/978-3-662-48005-2 .
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    Agusto, M. R.; Caselli, A.; Daga, R.; Varekamp, J.; Trinelli, A.; Dos Santos Afonso, M.; Velez, M. L.; Euillades, P.; Ribeiro Guevara, S. The crater lake of Copahue volcano (Argentina): geochemical and thermal changes between 1995 and 2015. In Geochemistry and Geophysics of Active Volcanic Lakes; Ohba, T., Capaccioni, B., Caudron, C., Eds.; Geological Society, London, Special Publications, 2016; Vol. 437; pp 107– 130.
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    Varekamp, J. C.; Ouimette, A. P.; Herman, S. W.; Flynn, K. S.; Bermudez, A.; Delpino, D. Naturally acid waters from Copahue volcano, Argentina. Appl. Geochem. 2009, 24, 208– 220,  DOI: 10.1016/j.apgeochem.2008.11.018
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    Naturally acid waters from Copahue volcano, Argentina
    Varekamp, J. C.; Ouimette, A. P.; Herman, S. W.; Flynn, K. S.; Bermudez, A.; Delpino, D.
    Applied Geochemistry (2009), 24 (2), 208-220CODEN: APPGEY; ISSN:0883-2927. (Elsevier Ltd.)
    Volcanic acid sulfate-chloride brines form through absorption of volcanic vapors in shallow reservoirs of meteoric water. Reaction with surrounding volcanic rocks leads to partial neutralization of the fluids and pptn. of secondary minerals. Chem. data of such acid waters from Copahue volcano, Argentina, covering 8 years of observations, show evidence for changes in compn. related to water rock interaction at depth prior to emergence of the fluids at the surface. The chem. compn. changed dramatically during the 2000 eruption of Copahue, with enhanced concns. and fluxes of Mg, Na, Fe and Al, followed in 2001 by rapidly declining concns. and element fluxes. The subsequent 5 years saw more variable element ratios and strong depletions in K and Al. Most incompatible elements are released from the rock matrix stochiometrically, whereas some elements are enriched through vapor input from the magma (As, Pb, Zn). Most fluids have LREE enrichments relative to the rock matrix, but during periods of new magma intrusion the LREE enrichment decreases as does the magnitude of the neg. Eu anomaly in the fluids. These observations are interpreted assuming early dissoln. of plagioclase, olivine and volcanic glass that occurs during intrusion of new magma into the hydrothermal system. The high field strength elements are virtually immobile even in these hot acid fluids, with Nb and Ta more so than Hf and Zr. The mobility of U and Th in these fluids is comparable, at variance with Th behavior in neutral fluids. The local rivers and lakes of Copahue are fertilized by volcanic dissolved P, and most surface waters with pH <3 have high levels of As. The acid fluids from Copahue may be surficial analogs for deep subduction fluids that evolve below zones of arc magma generation as well as for early Mars environments that are thought to have had large acid lakes.
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    Christenson, B. W.; Wood, C. P. Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake, New Zealand. Bull. Volcanol. 1993, 55, 547– 565,  DOI: 10.1007/BF00301808
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    Inguaggiato, C.; Censi, P.; Zuddas, P.; Londoño, J. M.; Chacón, Z.; Alzate, D.; Brusca, L.; D’Alessandro, W. Geochemistry of REE, Zr and Hf in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia). Chem. Geol. 2015, 417, 125– 133,  DOI: 10.1016/j.chemgeo.2015.09.025
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    Geochemistry of REE, Zr and Hf in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia)
    Inguaggiato, C.; Censi, P.; Zuddas, P.; Londono, J. M.; Chacon, Z.; Alzate, D.; Brusca, L.; D'Alessandro, W.
    Chemical Geology (2015), 417 (), 125-133CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    The geochem. behavior of Rare Earth Elements, Zr and Hf was investigated in the thermal waters of Nevado del Ruiz volcano system. A wide range of pH, between 1.0 and 8.8, characterizes these fluids. The acidic waters are sulfate dominated with different Cl/SO4 ratios. The important role of the pH and the ionic complexes for the distribution of REE, Zr and Hf in the aq. phase was evidenced. The pH rules the pptn. of authigenic Fe and Al oxyhydroxides producing changes in REE, Zr, Hf amts. and strong anomalies of Cerium. The pptn. of alunite and jarosite removes LREE from the soln., changing the REE distribution in acidic waters. Y-Ho and Zr-Hf (twin pairs) have a different behavior in strong acidic waters with respect to the water with pH near-neutral. Yttrium and Ho behave as Zr and Hf in waters with pH near neutral-to-neutral, showing super-chondritic ratios. The twin pairs showed to be sensitive to the co-pptn. and/or adsorption onto the surface of authigenic particulate (Fe-, Al-oxyhydroxides), suggesting an enhanced scavenging of Ho and Hf with respect to Y and Zr, leading to superchondritic values. In acidic waters, a different behavior of twin pairs occurs with chondritic Y/Ho ratios and sub-chondritic Zr/Hf ratios. For the first time, Zr and Hf were investigated in natural acidic fluids to understand the behavior of these elements in extreme acidic conditions and different major anion chem. Zr/Hf molar ratio changes from 4.75 to 49.29 in water with pH < 3.6. In strong acidic waters the fractionation of Zr and Hf was recognized as function of major anion contents (Cl and SO4), suggesting the formation of complexes leading to sub-chondritic Zr/Hf molar ratios.
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    Colvin, A.; Rose, W. I.; Varekamp, J. C.; Palma, J. L.; Escobar, D.; Gutierrez, E.; Montalvo, F.; Maclean, A.: Crater lake evolution at Santa Ana Volcano (El Salvador) following the 2005 eruption. In Understanding open-vent volcanism and related hazards: Geological Society of America Special Paper 498; Rose, W. I., Palma, J. L., Delgado Granados, H., Varley, N., Eds.; Geological Society of America, 2013; pp 23– 43.
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    Fernández-Remolar, D. C.; Morris, R. V.; Gruener, J. E.; Amils, R.; Knoll, A. H. The Río Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars. Earth Planet. Sci. Lett. 2005, 240, 149– 167,  DOI: 10.1016/j.epsl.2005.09.043
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    The Rio Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars
    Fernandez-Remolar, David C.; Morris, Richard V.; Gruener, John E.; Amils, Ricardo; Knoll, Andrew H.
    Earth and Planetary Science Letters (2005), 240 (1), 149-167CODEN: EPSLA2; ISSN:0012-821X. (Elsevier B.V.)
    Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Rio Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral pptn. and diagenesis, while informing considerations of martian astrobiol. Oxidn., thought to be biol. mediated, of pyritic ore bodies by groundwaters in the source area of the Rio Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evapn. of river water drives pptn. of hydronium jarosite and schwertmannite, while (Mg,Al,Fe3+)-copiapite, coquimbite, gypsum, and other sulfate minerals ppt. nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the pptn. of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or pptd. directly from groundwaters is not known. The retention of jarosite and other sol. sulfate salts suggests that water limited the diagenesis of Meridiani rocks. Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Rio Tinto environments. Org. matter does not persist in Rio Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiol. investigation of Meridiani outcrops.
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    Nordstrom, D. K.; Alpers, C. N.; Ptacek, K. J.; Blowes, D. W. Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California. Environ. Sci. Technol. 2000, 34, 254– 258,  DOI: 10.1021/es990646v
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    Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California
    Nordstrom, Darrell Kirk; Alpers, Charles N.; Ptacek, Carol J.; Blowes, David W.
    Environmental Science and Technology (2000), 34 (2), 254-258CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Extremely acidic mine waters with pH values as low as -3.6, total dissolved metal concns. as high as 200 g/L, and sulfate concns. as high as 760 g/L, have been encountered underground in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as -4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidn. and concn. by evapn. with minor effects from aq. ferrous iron oxidn. and efflorescent mineral formation.
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    Craig, H.: Geochemistry and origin of the Red Sea brines. In Hot brines and recent heavy metal deposits in the Red Sea, A geochemical and geophysical acoount; Degens, E. T., Ross, D. A., Eds.; Springer: Berlin, Heidelberg, 1969; pp 208– 242.
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    The thermodynamics and kinetics of the hydrogen sulfide system in natural waters
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    Marine Chemistry (1986), 18 (2-4), 121-47CODEN: MRCHBD; ISSN:0304-4203.
    A review with 81 refs. on the thermodn. and kinetics of the H2S system in natural waters. Equations are derived relating the soly. and ionization of H2S in water and seawater as a function of salinity, temp., and pressure. The limited data available for the interaction of trace metals for HS- are summarized. The kinetics of oxidn. of H2S is examd. as a function of pH, temp., and salinity. The discrepancies in the available data are largely due to the different [O2]/[HS-] ratios used in various studies. Over a limited pH range (6-8) the pseudo first order rate const. for the oxidn. is directly proportional to the activity of HS-.
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    A spectrophotometric study of iron(III) hydrolysis in aqueous solutions to 200°C
    Stefansson, Andri; Seward, Terry M.
    Chemical Geology (2008), 249 (1-2), 227-235CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    The hydrolysis of iron(III) was studied in acid aq. solns. between 25 and 200° at satd. water vapor pressure by uv-vis spectrophotometry using a high-temp., flow-through gold-lined optical cell. The strong ligand-to-metal charge transitions of iron(III) hydroxo complexes at wavelengths below 400 nm were used to obtain molar absorptivities, ε, and equil. hydrolysis consts. using principal component anal. of the spectra. The total iron(III) concns. ranged from 6.184 × 10-5 to 1.652 × 10-4 mol/kg-1 and the perchloric acid concn. was between 9.31 × 10-4 and 0.398 mol/kg-1. Under these conditions two iron(III) species were identified, Fe3+ and FeOH2+, and the corresponding hydrolysis const. according to the reaction increased from logβ1 = - 2.18 ± 0.01 at 25° to 0.54 ± 0.15 at 200°. The hydrolysis of Fe3+ to form FeOH2+ is predominantly driven by pos. entropy indicating electrostatic interaction between Fe3+ and OH-.
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    Kanno, H.; Hiraishi, J. A Raman study of aqueous solutions of ferric nitrate, ferrous chloride and ferric chloride in the glassy state. J. Raman Spectrosc. 1982, 12, 224– 227,  DOI: 10.1002/jrs.1250120305
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    A Raman study of aqueous solutions of ferric nitrate, ferrous chloride and ferric chloride in the glassy state
    Kanno, H.; Hiraishi, J.
    Journal of Raman Spectroscopy (1982), 12 (3), 224-7CODEN: JRSPAF; ISSN:0377-0486.
    Raman spectra were obtained for aq. solns. of Fe(NO3)3, FeCl2, and FeCl3 in the glassy state where R = moles of water/mol of salt = 10-20. The predominant ionic species in these solns. are [Fe(H2O)6]3+, [Fe(H2O)6]2+, and trans-[FeCl2(H2O)4]+, resp. The 2nd dominant ferric species in the glassy FeCl3 soln. (R = 20) seems to be the [FeCl(H2O)5]2+ ion, which is inferred from comparison of the frequencies of the Fe-OH2 stretching Raman bands.
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    Koyama, Y.; Umemoto, Y.; Akamatsu, A.; Uehara, K.; Tanaka, M. Raman spectra of chrolophyll forms. J. Mol. Struct. 1986, 146, 273– 287,  DOI: 10.1016/0022-2860(86)80299-X
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    Withnall, R.; Chowdhry, B. Z.; Edwards, S. J. Raman spectra of carotenoids in natural products. Spectrochim. Acta, Part A 2003, 59, 2207– 2212,  DOI: 10.1016/S1386-1425(03)00064-7
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    Raman spectra of carotenoids in natural products
    Withnall, Robert; Chowdhry, Babur Z.; Silver, Jack; Edwards, Howell G. M.; de Oliveira, Luiz F. C.
    Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2003), 59A (10), 2207-2212CODEN: SAMCAS; ISSN:1386-1425. (Elsevier Science B.V.)
    Resonance Raman spectra of naturally occurring carotenoids have been obtained from nautilus, periwinkle (Littorina littorea) and clam shells under 514.5 nm excitation and these spectra are compared with the resonance Raman spectra obtained in situ from tomatoes, carrots, red peppers and saffron. The tomatoes, carrots and red peppers gave rise to resonance Raman spectra exhibiting a ν1 band at ∼1520 cm-1, in keeping with its assignment to carotenoids with ∼nine conjugated carbon-carbon double bonds in their main chains, whereas the resonance Raman spectrum of saffron showed a ν1 band at 1537 cm-1 which can be assigned to crocetin, having seven conjugated carbon-carbon double bonds. A correlation between ν1 wavenumber location and effective conjugated chain length has been used to interpret the data obtained from the shells, and the wavenumber position (1522 cm-1) of the ν1 band of the carotenoid in the orange clam shell suggests that it contains nine conjugated double bonds in the main chain. However, the black periwinkle and nautilus shells exhibit ν1 bands at 1504 and 1496 cm-1, resp. On the basis of the correlation between ν1 wavenumber location and effective conjugated chain length, this indicates that they contain carotenoids with longer conjugated chains, the former having ∼11 double bonds and the latter ∼13 or even more. Raman spectra of the nautilus, periwinkle and clam shells also exhibited a strong band at 1085 cm-1 and a doublet with components at 701 and 705 cm-1, which can be assigned to biogenic calcium carbonate in the aragonite crystallog. form.
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    Nugent, P. W.; Shaw, J. A.; Vollmer, M. Colors of thermal pools at Yellowstone National Park. Appl. Opt. 2015, 54, B128– 139,  DOI: 10.1364/AO.54.00B128
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    Isotopic variations in meteoric waters
    Craig, Harmon
    Science (Washington, DC, United States) (1961), 133 (), 1702-3CODEN: SCIEAS; ISSN:0036-8075.
    The relation between deuterium and O18 concns. in natural meteoric waters from many parts of the world was detd. with a mass spectrometer. The isotopic enrichments, relative to that of ocean water, display a linear correlation over the entire range for waters that have not undergone excessive evapn.
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    Sofer, Z.; Gat, J. The isotope composition of evaporating brines: effect of the isotopic activity ratio in saline solutions. Earth Planet. Sci. Lett. 1975, 26, 179– 186,  DOI: 10.1016/0012-821X(75)90085-0
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    Isotope composition of evaporating brines. Effect of the isotopic activity ratio in saline solutions
    Sofer, Z.; Gat, J. R.
    Earth and Planetary Science Letters (1975), 26 (2), 179-86CODEN: EPSLA2; ISSN:0012-821X.
    A math. expression is given for the ratio of the activity coeffs. of deuterated and light water in chloridic saline solns. as a function of the molality of the salt components. Calcns. based on the Craig-Gordon evapn. model show that in the case of evapg. brines, the salt effect on the activity coeffs. of the isotopic water results in deviation of the δ values from a const. slope evapn. line, when plotted on the conventional δ-diagram, of D-concn. vs. 18O activity. Conversion of concn. data to activities eliminates these deviations to a large extent, indicating them to be an artifact of the scales employed for measuring the isotopic abundance. The origin of Dead Sea waters and other natural brines is discussed in the light of these findings.
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    Oerter, E. J.; Singleton, M.; Davisson, M. L. Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions. Geochim. Cosmochim. Acta 2018, 238, 316– 328,  DOI: 10.1016/j.gca.2018.07.009
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    Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions
    Oerter, Erik J.; Singleton, Michael; Davisson, M. Lee
    Geochimica et Cosmochimica Acta (2018), 238 (), 316-328CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    An attractive tracer of water-surface interactions in humidity exposure expts. are the stable hydrogen and oxygen isotopes in water vapor, though the conventional method of using salt satd. aq. solns. to control humidity is likely to exert strong effects on the H and O stable isotope compn. of the vapor. The magnitude of these effects are virtually unknown for hyper-saline solns. near salt satn. and beyond. Here we explore the hydrogen and oxygen stable isotope effects of salt-water mixts. from dil. to the salt satn. point and beyond for BaCl2, CaCl2, KCl, K2SO4, MgCl2, NaBr, NaCl, and NaI salts, of which the O isotope results on BaCl2, K2SO4, and NaBr are the first to be reported, and the first for H with BaCl2. We find that vapor humidity levels, and hydrogen and oxygen isotope effects in the vapor generated from dil. solns. to salt-satd. mixts. increase in magnitude by predictable linear trends. In the case of hydrogen isotope salt effects, the cation identity matters, as the effect becomes stronger with decreasing size and increasing charge (increasing ionic potential) in the sequence of: K < Na < Ba < Ca < Mg. For oxygen, the cation effect becomes stronger with increasing ionic potential, in the order of Na < Ba < K < Ca < Mg (irresp. of sign). Hydrogen and oxygen isotope effects in vapor from salt concns. greater than the salt satn. point do not behave as systematically, with some of the salts displaying no increase in isotope effects beyond satn., while others show an increase in isotope effects beyond salt satn., esp. in the case of hydrogen. We make specific recommendations on the salts that exert hydrogen and oxygen isotope effects with the least magnitude and most predictability for humidity levels from 26% to 92%. The use of salt solns. to create vapor at less than 100% and with specific δ2H and δ18O values that differ from that defined by the global meteoric water relationship also seems possible. Hyper-saline surface waters can exist at and above satn., and thus these isotope effects may be present in them, as well as in minerals that form from these waters.
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    Snyder, G.; Poreda, R.; Fehn, U.; Hunt, A. Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components. Geochem., Geophys., Geosyst. 2003, 4, 1– 28,  DOI: 10.1029/2002GC000363
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    Raven, J. A.; Walker, D. I.; Johnston, A. M.; Handley, L. L. Implications of 13 C natural abundance measurements for photosynthetic performance by marine macrophytes in their natural environment. Mar. Ecol.: Prog. Ser. 1995, 123, 193– 205,  DOI: 10.3354/meps123193
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    Sheppard, S. M.; Nielsen, R. L.; Taylor, H. P. Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits. Econ. Geol. Bull. Soc. Econ. Geol. 1971, 66, 515– 542,  DOI: 10.2113/gsecongeo.66.4.515
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    Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits
    Sheppard, Simon M. F.; Nielsen, Richard L.; Taylor, Hugh P., Jr.
    Economic Geology and the Bulletin of the Society of Economic Geologists (1971), 66 (4), 515-42CODEN: ECGLAL; ISSN:0361-0128.
    The 18O/16O, D/H, and 13C/12C values were measured on quartz, K feldspar, biotite, sericite, and clcite from potassic, sericitic, and argillic alteration assemblages and from fresh igneous rocks and veins in 9 North American porphyry Cu and Mo deposits, and in 4 other hydrothermal mineral deposits. Both sericites and biotites preserve their 18O/16O and D/H values, as these isotope ratios are not affected by later low-temp. exchange with local ground waters. A systematic correlation between the D/H ratios of sericites and those of meteoric ground waters indicates the presence of meteoric waters in the hydrothermal fluids involved in sericitization, whereas the restricted range of D/H ratios of biotites indicates the probable dominance of magmatic waters in the fluids assocd. with biotite alteration. The O isotope data for the quartz-K feldspar-biotite-chalcopyrite assemblages also support a high temp. of formation in the presence of waters with δ18O values in the magmatic water range. 13C/12C values of hydrothermal vein carbonates in the porphyry Cu deposits (δ = -2.6 to -5.9) tend to be a little heavier than primary igneous calcites from carbonatites (δ = -5 to -8). It is suggested that heated Na-Ca-Cl brines originally present in the neighboring sedimentary and volcanic sections may be an important component of hydrothermal fluids. The Santa Rita, N. Mex., porphyry Cu deposit isotopic data are discussed in detail.
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    Sheppard, S. M. F. Characterization and Isotopic Variations in natural waters. Reviews in Minerology 1986, 16, 165– 183. DOI: 10.1515/9781501508936-011
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    Rye, R. O.; O’Neil, J. R. The 18O content of water in primary fluid inclusions from Providencia, north-central Mexico. Econ. Geol. 1968, 63, 232– 238,  DOI: 10.2113/gsecongeo.63.3.232
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    The oxygen-18 content of water in primary fluid inclusions from Providencia, north-central Mexico
    Rye, Robert O.; O'Neil, James R.
    Economic Geology and the Bulletin of the Society of Economic Geologists (1968), 63 (3), 232-8CODEN: ECGLAL; ISSN:0361-0128.
    Water and CO2 extd. from inclusions in sphalerite, calcite, and quartz were analyzed for 18O and 13C. The δ18O of inclusions in sphalerite from 3 different ore bodies ranged 6.2-5.8‰ relative to Standard Mean Ocean Water. These values are lower than the 7-9‰ believed to be typical for pristine magmatic water, and have probably resulted from isotopic exchange between hydrothermal fluids and the cryst. granodiorite through which fluids passed. The δ18O of water in inclusions in calcite and quartz indicate that the inclusions have exchanged 18O with their host during the cooling of the ore deposit, but less than 45% of the exchange that is possible at surface temps. has occurred. The δ13C of CO2 in inclusions in sphalerite and calcite was -7.0 and -11.0‰ relative to PDB. The sphalerite value is consistent with the value previously inferred for δ13C of the hydrothermal fluid, and is believed to be typical of juvenile CO2. The calcite reflects ∼50% of the possible exchange that could occur between inclusion CO2 and the host at the surface.
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    Rothschild, L. J.; Mancinelli, R. L. Life in extreme environments. Nature 2001, 409, 1092– 1101,  DOI: 10.1038/35059215
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    Life in extreme environments
    Rothschild, Lynn J.; Mancinelli, Rocco
    Nature (London, United Kingdom) (2001), 409 (6823), 1092-1101CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Each recent report of liq. water existing elsewhere in the Solar System has reverberated through this international press and excited the imagination of humankind. Why Because in the past few decades we have come to realize that where there is liq. water on Earth, virtually no matter what the phys. conditions, there is life. What we previously thought of as insurmountable phys. and chem. barriers to life, we now see as yet another niche harbouring 'extremophiles'. This realization, coupled with new date on the survival of microbes in the space environment and modeling of the potential for transfer of life between celestial bodies, suggests that life could be more common than previously thought. Here we examine critically what it means to be an extremophile, and the implications of this for evolution, biotechnol. and esp. the search for life in the Universe.
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    Harrison, J. P.; Gheeraert, N.; Tsigelnitskiy, D.; Cockell, C. S. The limits for life under multiple extremes. Trends Microbiol. 2013, 21, 204– 212,  DOI: 10.1016/j.tim.2013.01.006
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    The limits for life under multiple extremes
    Harrison, Jesse P.; Gheeraert, Nicolas; Tsigelnitskiy, Dmitry; Cockell, Charles S.
    Trends in Microbiology (2013), 21 (4), 204-212CODEN: TRMIEA; ISSN:0966-842X. (Elsevier Ltd.)
    A review. Life on Earth is limited by phys. and chem. extremes that define the 'habitable space' within which it operates. Aside from its requirement for liq. water, no definite limits were established for life under any extreme. Here, we employ growth data published for 67 prokaryotic strains to explore the limitations for microbial life under combined extremes of temp., pH, salt (NaCl) concns., and pressure. Our review reveals a fundamental lack of information on the tolerance of microorganisms to multiple extremes that impedes several areas of science, ranging from environmental and industrial microbiol. to the search for extraterrestrial life.
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    Bekker, A.; Holland, H. D.; Wang, P. L.; Rumble, D., Iii; Stein, H. J.; Hannah, J. L.; Coetzee, L. L.; Beukes, N. J. Dating the rise of atmospheric oxygen. Nature 2004, 427, 117,  DOI: 10.1038/nature02260
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    Dating the rise of atmospheric oxygen
    Bekker, A.; Holland, H. D.; Wang, P.-L.; Rumble, D.; Stein, H. J.; Hannah, J. L.; Coetzee, L. L.; Beukes, N. J.
    Nature (London, United Kingdom) (2004), 427 (6970), 117-120CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Several lines of geol. and geochem. evidence indicate that the level of atm. oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels by 2.22 Gyr ago. Here we present evidence that the rise of atm. oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in org.-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic compn. of sulfur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atm. oxygen was present at significant levels (i.e., greater than 10-5 times that of the present atm. level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of heamatitic pisolites and ooelites in the upper Timeball Hill Formation indicate that atm. oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Paleoproterozoic glacial events.
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    Lyons, T. W.; Reinhard, C. T.; Planavsky, N. J. The rise of oxygen in Earth’s early ocean and atmosphere. Nature 2014, 506, 307– 315,  DOI: 10.1038/nature13068
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    The rise of oxygen in Earth's early ocean and atmosphere
    Lyons, Timothy W.; Reinhard, Christopher T.; Planavsky, Noah J.
    Nature (London, United Kingdom) (2014), 506 (7488), 307-315CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    The rapid increase of carbon dioxide concn. in Earth's modern atm. is a matter of major concern. But for the atm. of roughly two-and-half billion years ago, interest centers on a different gas: free oxygen (O2) spawned by early biol. prodn. The initial increase of O2 in the atm., its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.
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    Halevy, I.; Alesker, M.; Schuster, E. M.; Popovitz-Biro, R.; Feldman, Y. A key role for green rust in the Precambrian oceans and the genesis of iron formations. Nat. Geosci. 2017, 10, 135– 139,  DOI: 10.1038/ngeo2878
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    A key role for green rust in the Precambrian oceans and the genesis of iron formations
    Halevy, I.; Alesker, M.; Schuster, E. M.; Popovitz-Biro, R.; Feldman, Y.
    Nature Geoscience (2017), 10 (2), 135-139CODEN: NGAEBU; ISSN:1752-0894. (Nature Publishing Group)
    Iron formations deposited in marine settings during the Precambrian represent large sinks of iron and silica, and have been used to reconstruct environmental conditions at the time of their formation. However, the obsd. mineralogy in iron formations, which consists of iron oxides, silicates, carbonates and sulfides, is generally thought to have arisen from diagenesis of one or more mineral precursors. Ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates. Here we investigate the potential role of green rust, a ferrous-ferric hydroxy salt, in the genesis of iron formations. Our lab. expts. show that green rust readily forms in early seawater-analog solns., as predicted by thermodn. calcns., and that it ages into minerals obsd. in iron formations. Dynamic models of the iron cycle further indicate that green rust would have pptd. near the iron redoxcline, and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments. We suggest, therefore, that the pptn. and transformation of green rust was a key process in the iron cycle, and that the interaction of green rust with various elements should be included in any consideration of Precambrian biogeochem. cycles.
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    Canfield, D. E. The early history of atmospheric oxygen: Homage to Robert M. Garrels. Annu. Rev. Earth Planet. Sci. 2005, 33, 1– 36,  DOI: 10.1146/annurev.earth.33.092203.122711
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    The early history of atmospheric oxygen: Homage to Robert M. Garrels
    Canfield, D. E.
    Annual Review of Earth and Planetary Sciences (2005), 33 (), 1-36CODEN: AREPCI; ISSN:0084-6597. (Annual Reviews Inc.)
    A review. This paper reviews the Precambrian history of atm. oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atm. oxygen concns. Also reviewed are the various processes regulating oxygen concns., and several models of Precambrian oxygen evolution are presented. A sparse geol. record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atm. oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.
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    Klingelhofer, G.; Morris, R. V.; Bernhardt, B.; Schroder, C.; Rodionov, D. S.; de Souza, P. A., Jr.; Yen, A.; Gellert, R.; Evlanov, E. N.; Zubkov, B.; Foh, J.; Bonnes, U.; Kankeleit, E.; Gutlich, P.; Ming, D. W.; Renz, F.; Wdowiak, T.; Squyres, S. W.; Arvidson, R. E. Jarosite and hematite at Meridiani Planum from Opportunity’s Mössbauer Spectrometer. Science 2004, 306, 1740– 1745,  DOI: 10.1126/science.1104653
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    Jarosite and hematite at Meridiani Planum from Opportunity's Mossbauer Spectrometer
    Klingelhofer G; Morris R V; Bernhardt B; Schroder C; Rodionov D S; de Souza P A Jr; Yen A; Gellert R; Evlanov E N; Zubkov B; Foh J; Bonnes U; Kankeleit E; Gutlich P; Ming D W; Renz F; Wdowiak T; Squyres S W; Arvidson R E
    Science (New York, N.Y.) (2004), 306 (5702), 1740-5 ISSN:.
    Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.
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    Amils, R.; Fernandez-Remolar, D. Rio Tinto: a geochemical and mineralogical terrestrial analogue of Mars. Life (Basel, Switz.) 2014, 4, 511– 534,  DOI: 10.3390/life4030511
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    Rio Tinto: a geochemical and mineralogical terrestrial analogue of mars
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    Life (Basel, Switzerland) (2014), 4 (3), 511-534CODEN: LBSIB7; ISSN:2075-1729. (MDPI AG)
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Published December 6, 2018

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  • Abstract

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    Figure 1

    Figure 1. The hydrothermal system of Dallol within the Erta Ale range and maps of the active hydrothermal features of January 2017. (a) Dallol dome located in Danakil depression within the Erta Ale volcanic range of the Afar Triangle. (b) Geological map of the wider area of the Dallol dome, including Black and Yellow Lakes. (c) Aerial image of Dallol dome, showing the active areas in January 2017. (d) Map of January 2017 of the hydrothermal activity showing the referred sampling sites within the text, spring (S1), gray spring (S3) and a system of four successive pools (P1–P4).

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    Figure 2

    Figure 2. Representative image of the hydrothermal system of Dallol. Fumaroles and active hydrothermal chimneys forming terraces and pools are shown at the background of the image. Moving downward from the terraces, the oxidation of the Fe mineral precipitates is manifested by the change in the color from white to green, yellow, and finally to red. Pillars formed by exhausted springs of past hydrothermal activity are shown at the upper left and right of the image. The height of the right pillar is 1.5 m.

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    Figure 3

    Figure 3. Comparative bar graph of sulfate, chloride, fluoride, and iron concentrations of Dallol (purple-shaded area) and other hyperacidic volcanic sites and the Red Sea brines. The values for Copahue, (22) Poas, (17,18) Ruapehu, (23) Nevado del Ruiz, (24) Ebeko, (19) Santa Ana, (25) Kawah Ijen, (16) Red Sea brines, (28) and Dallol (this study) acidic fluids are shown in ppm.

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    Figure 4

    Figure 4. Fe(II)/Fe(III) ratio, pH, O2 concentration, and temperature of four successive pools of a typical spring-terrace system. Pool 1 (P1), the upper pool (oxygen-free spring is at the left of the pool, out of the field of view), is dominated by Fe(II) species of light green color. Moving downstream from the spring to pool 3 (P3), temperature is decreasing and the atmospheric O2 diffusion results to the formation of Fe(III) species and to pH drop. Ultimately, the lowest pool 4 (P4) is characterized by Fe(III), the precipitation of which darkens the color of the pool, lowers further the pH, increases the temperature, and consumes the oxygen.

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    Figure 5

    Figure 5. Raman and UV–vis spectra of the spring water (S1) and the four gradual pools (P1–P4) of Figure 1 and Figure 4. (a) Region of the Raman spectra showing the main vibrations of the Fe aqueous complexes. The concentration of the Fe(III) is increasing gradually from pool 2 to pool 4, as shown by the Fe(III)-Cl stretching vibration band at 312 cm–1. This correlates with the decrease of the intensity of the Fe(II)–OH2 stretching vibration at 370 cm–1. (b) UV-absorbance spectra showing the increase of the band centered at 330 nm from the spring and upper pool to the lowest pools, due to Fe(II) oxidation and Cl-complexation.

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    Figure 6

    Figure 6. Pictures, microscopic images, and mineral composition of different Dallol patterns. (a) Water-lily structures created by subaqueous hydrothermal activity. (b) Halite flowers with variable degree of oxidation. (c) Halite with egg-like shapes forming around gas vents. (d) Twisted hollow tubes of halite. (e) Halite pearls. (f) Efflorescences. (g) Chocolate bar cracks formed during crystallization of an Fe-rich brine or melt. (h) FESEM and Fe, K, S, Cl EDX map of a jarosite-rich efflorescence. (i) FESEM image of jarosite spherules. (j) FESEM image of part of a jarosite spherule decorated by akaganeite particles. (k) TEM/HAADF image of the akaganeite nanoparticles. (l) hematite spindles. (m) Micro-Raman spectrum of the hematite spindles. Mineral abbr.: Hl=Halite, Jrs=Jarosite, Na-Jrs=Natrojarosite, Ak=Akaganeite, Gt=Goethite, Fe-OH=Fe-oxyhydroxides, Fe-O=Fe-oxides, Hem=Hematite, Syl=Sylvite, Gp=Gypsum, Anh=Anhydrite, Car=Carnallite, S=Sulfur, Fe-/Mg-/Ca-Sil=Fe-/Mg-/Ca-silicates.

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    Figure 7

    Figure 7. Stable isotope data for the Dallol hydrothermal brines and Black and Yellow Lake waters (2016 and 2017 campaigns). Dallol spring water has δD‰ values similar to the local meteoric water. The anomalous high δ18O values of the hot springs indicate the interaction of the meteoric water with the underlying basaltic flows and evaporites. The δ18O‰ and δD values of the Dallol pools are higher due to evaporation processes (δ18O > +10 ‰, δD > −10 ‰). Yellow and Black Lake waters exhibit relatively negative values for both δD (−25‰ to −48‰) and δ18O (−1‰ to −7‰) showing distinct hydrological processes with respect to the Dallol dome. MWL, Meteoric Water Line; (41) AAMWL, Addis Ababa Meteoric Water Line. (42) Magmatic and primary magmatic waters are plotted after refs (48−50.)

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      13
      Gas chemistry of the Dallol region of the Danakil Depression in the Afar region of the northern-most East African Rift
      Darrah, Thomas H.; Tedesco, Dario; Tassi, Franco; Vaselli, Orlando; Cuoco, Emilio; Poreda, Robert J.
      Chemical Geology (2013), 339 (), 16-29CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
      A combination of noble and major gas compn. and isotope geochem. provides a window into the source of volatiles and the mechanisms of transport assocd. with a series of hot springs located near the Dallol volcano within the Danakil Depression along the Red Sea arm of the Afar triple junction. The helium isotopic compn. of these gases range up to 11.9 times the atm. ratio 11.9 R/Ra, which suggests that the Afar plume interacts with the Afar depression across at least the 300 km transect from Tendaho-Gabo basin to Dallol within the Danakil Depression. The 4He/40Ar* of ~ 14 in the mantle-rich end-member at Dallol indicates significant degassing prior to emplacement at Dallol either during basaltic dike intrusions beneath the Danakil Depression or during the release and transport of fluids from a degassed subsolidus source in the upper mantle along high permeability fracture zones. The CO2/3He of the magmatic end-member is ~ 2 × higher 7.7 × 109 and more pos. with δ13C CO2 -2.1‰ than other archetypal plumes e.g. Hawaii, Iceland, etc.. The Dallol compn. is consistent with a hypothetical model that assumes a plume-type starting compn. and experiences ~ 92% degassing (where helium is preferentially degassed with respect to CO2) and the addn. of CO2 from the thermal degrdn. of carbonate. Non-atm. excess N2 with a δ15N (N2) of + 3.5 to + 4‰ dominates the Dallol volatiles and suggests interaction between mantle fluids and Proterozoic meta-sediments. By comparing and modeling the range in atmospherically e.g. 20Ne, 36Ar, 84Kr and mantle-derived e.g. 4He/40Ar* and CO2/3He components in Dallol volatiles, we propose that the coherent variations in these gases result from mixing of magmatic volatiles with extremely degassed remnant fluids present within the hydrothermal reservoir.
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    14. 14
      Bonatti, E.; Fisher, D. E.; Joensuu, O.; Rydell, H. S.; Beyth, M. Iron-Manganese-Barium Deposit from the Northern Afar Rift (Ethiopia). Econ. Geol. Bull. Soc. Econ. Geol. 1972, 67, 717– 730,  DOI: 10.2113/gsecongeo.67.6.717
      14
      Iron-manganese-barium deposit from the northern Afar Rift (Ethiopia)
      Bonatti, Enrico; Fisher, David E.; Joensuu, Oiva; Rydell, Harold S.; Beyth, Michael
      Economic Geology and the Bulletin of the Society of Economic Geologists (1972), 67 (6), 717-30CODEN: ECGLAL; ISSN:0361-0128.
      The Afar depression is a continuation of the Red Sea rift, and the deposit is similar to the hot brine metalliferous deposits from the Red Sea and the deposits from active oceanic rifts. The deposit was formed about 200,000 yr ago as a result of submarine hydrothermal activity whereby solns. rich in Fe, Mn, Ba, Si, and other elements were injected through the sea floor and fractional pptn. of these elements followed. Fe-rich basal layers with goethite and nontronite as principal constituents are overlain by Mn-rich strata. Some layers approach a pure pyrolusite ore, and some are characterized by ≤6% Ba in the form of crystals of strontiobarite contg. ≤12% SrSO4 in solid soln. The concn. of Ni, Co, Cr, and Cu is very low when compared with marine sedimentary Fe-Mn deposits. The U and Th isotopic compn. of the deposit indicates that the Fe and Mn minerals were not deposited from normal sea water.
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    15. 15
      Sharp, Z. D.; Atudorei, V.; Durakiewicz, T. A rapid method for determination of hydrogen and oxygen isotope ratios from water and hydrous minerals. Chem. Geol. 2001, 178, 197– 210,  DOI: 10.1016/S0009-2541(01)00262-5
      15
      A rapid method for determination of hydrogen and oxygen isotope ratios from water and hydrous minerals
      Sharp, Z. D.; Atudorei, V.; Durakiewicz, T.
      Chemical Geology (2001), 178 (1-4), 197-210CODEN: CHGEAD; ISSN:0009-2541. (Elsevier Science B.V.)
      A general-purpose, online, continuous flow method for detn. of δD and δ18O values of water and hydrous minerals is described. Minor modifications of com. available equipment allow for analyses of water and solid samples, fluid inclusions and in situ hydrogen isotope detns. of hydrous minerals using a laser. The technique involves redn. of H2O or solid hydrous samples by reaction with glassy carbon at high temps. H2 and CO are produced by reaction with the carbon at 1450°C in a helium carrier gas. Product gases are sepd. in a gas chromatograph and analyzed in a mass spectrometer configured to make hydrogen isotope analyses in continuous flow mode. Sample size is as small as 0.1 μl of water (or equiv. from hydrous phases) for both hydrogen and oxygen isotope ratio detns. Waters are injected through a heated septa into the He stream; solid materials are wrapped in silver foil and dropped into the furnace using an autosampler. Using std. correction procedures, results obtained with this method are identical to those obtained conventionally with a precision for water samples of ±2‰ (1σ) for hydrogen and ±0.2‰ (1σ) for oxygen. Saline waters can be analyzed without any addnl. prepn. Reproducibility of δD values from hydrous silicates is also ±2‰. The δ18O values of 'dehydration' water evolved from biotite during heating is variable and irreproducible. Total time of anal. is less than 2 min for a single hydrogen isotope anal. Sample size can be reduced an order of magnitude by using a low He-flow rate, a narrow-bore redn. column, a capillary GC column and a more efficient open split. With the high sensitivity design, in situ measurements can be made on hydrous minerals using a CO2 laser for sample heating. Stable isotope detns. of fluid inclusions can be made by decrepitating samples in the He-stream.
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    16. 16
      Delmelle, P.; Bernard, A. Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia. J. Volcanol. Geotherm. Res. 2000, 97, 55– 75,  DOI: 10.1016/S0377-0273(99)00159-6
      16
      Downstream composition changes of acidic volcanic waters discharged into the Banyupahit stream, Ijen caldera, Indonesia
      Delmelle, P.; Bernard, A.
      Journal of Volcanology and Geothermal Research (2000), 97 (1-4), 55-75CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier Science B.V.)
      The crater lake of Kawah Ijen volcano contains extremely low pH (<0.4) waters with high SO4 (∼70000 mg/kg), Cl (∼21000 mg/kg), F (∼1500 mg/kg), Al (∼5000 mg/kg), Fe (∼2000 mg/kg) and trace metal (Cu ∼0.5, Zn ∼4, Pb ∼3 mg/kg) contents. These brines seep outward through the western crater rim and reappear on the other side as streamlets, which form the headwaters of the Banyupahit stream. The Banyupahit first mixes with fresh rivers and thermal springs in the Ijen caldera and then irrigates a coastal agricultural plain which is 30 km from the summit crater; downstream compn. changes affecting the Banyupahit waters are investigated using stable isotope, chem. and mineralogical data collected from sites along the stream length. The satn. of the stream waters with respect to minerals was evaluated with SOLVEQ and WATEQ4F and compared with the geochem. observations. An aluminous mineralogy (alunogen, pickeringite, tamarugite and kalinite) develops in the upper part of the Banyupahit due to concn. of the headwaters by evapn. Downstream attenuation of dissolved element concns. results principally from diln. and from mineral pptn. The stream pH changes from ∼0 at the source to °4 close to the mouth. The δD and δ18O values and the relative SO4-Cl-F contents of the Banyupahit waters indicate that the tributaries are mostly meteoric. Dissolved SO4 in the acidic stream come only from the crater lake seepages and are not involved later in microbially mediated reactions, as shown by their δ34S and δ18O values. Re-equilibration of the stream SO4 oxygen-isotope compn. with H2O from tributaries does not occur.Calcium, SiO2, Al, Fe, K and SO4 behave non-conservatively in the stream waters. Gypsum, silica (amorphous or poorly ordered), a basic aluminum hydroxysulfate (basaluminite), K-jarosite and amorphous ferric hydroxide may exert a soly. control on these elements along the entire stream length, or in certain stream sections, consistent with the thermochem. model results. Downstream concn. trends and mineral satn. levels suggest that pptn. of Sr-, Pb-rich barite and celestite consume Ba, Sr and Pb, whereas dissolved Cu, Pb and Zn may adsorb onto solid particles, esp. after the junctions of the acidic stream with non-acidic rivers. Significant fluxes of SO4, F, Cl, Al, SiO2, Ti, Mn and Cu may reach the irrigation system, possibly causing serious environmental impacts such as soil acidification and induration.
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    17. 17
      Rowe, G. L.; Brantley, S. L.; Fernandez, M.; Fernandez, J. F.; Borgia, A.; Barquero, J. Fluid-volcano interaction in an active stratovolcano: the crater lake system of Poás volcano, Costa Rica. J. Volcanol. Geotherm. Res. 1992, 49, 23– 51,  DOI: 10.1016/0377-0273(92)90003-V
      17
      Fluid-volcano interaction in an active stratovolcano: The crater lake system of Poas volcano, Costa Rica
      Rowe, Gary L., Jr.; Brantley, Susan L.; Fernandez, Mario; Fernandez, Jose F.; Borgia, Andrea; Barquero, Jorge
      Journal of Volcanology and Geothermal Research (1992), 49 (1-2), 23-51CODEN: JVGRDQ; ISSN:0377-0273.
      Seismic and geochem. data collected at Poas volcano, Costa Rica, since 1978 suggest that temp. and chem. variations recorded in subaerial fumaroles and the crater lake are related to episodic release of heat and volatiles assocd. with hydrofracturing of the upper margin of the shallow magma body. Power outputs assocd. with these events approach 600 MW and are superimposed on a baseline energy flux of approx. 200 MW. The baseline heat flux suggests a magma solidification rate of 0.012 km3/yr and background volatile release rates of 1000 H2O, 66, S, 13 Cl, and 0.5 tons/day F. These fluxes are comparable to fluxes of F, Cl, and S exiting the magmatic/hydrothermal system through acidic flank springs and are about a factor of five less than estd. volatile fluxes through summit fumaroles during the high-temp. event of 1981-1983. Mass balance considerations suggest that heat released at the lake surface is primarily supplied by the ascent of heated brines supplemented by condensation of fumarolic steam. Calcd. av. seepage rates out of the lake (≈450 kg/s) indicate rapid convection of acidic lake brine through underlying lake sediments and pyroclastic deposits. Circulation of the extremely corrosive lake brine through the volcanoclastic material beneath the lake may enhance subsurface permeabilities. Fluxes of rock-forming elements obsd. in the Rio Agrio basin on the northwest flank of Poas suggest that dissoln. and removal of volcano-clastic material occurs at a rate of approx. 1650 m3/yr. However, estd. sulfur fluxes from the cooling magma body suggest that porosity created by this dissoln. could quickly be filled by the deposition of native sulfur. Finally, anal. of heat and water budgets for the crater lake over the period 1978-1989 indicates that the recent decline and disappearance of the lake was caused by variations in summit rainfall combined with an increase in subsurface heat flow to the lake in 1987-1988. The increase in heat flow is related to continued degassing of high-temp. volatiles following a hydrofracturing/magma ascent episode that occurred in 1986.
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    18. 18
      Rodríguez, A.; van Bergen, M. J. Superficial alteration mineralogy in active volcanic systems: An example of Poás volcano, Costa Rica. J. Volcanol. Geotherm. Res. 2017, 346, 54– 80,  DOI: 10.1016/j.jvolgeores.2017.04.006
      18
      Superficial alteration mineralogy in active volcanic systems: An example of Poa´s volcano, Costa Rica
      Rodriguez, Alejandro; van Bergen, Manfred J.
      Journal of Volcanology and Geothermal Research (2017), 346 (), 54-80CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier B.V.)
      The alteration mineralogy in the crater area of Poa´s volcano (Costa Rica) has been studied to constrain acid fluid-rock interaction processes and conditions relevant for the formation of sulfate-bearing mineral assemblages found on the surface of Mars. Individual sub-environments, which include the hyperacid lake (Laguna Caliente), ephemeral hot springs, fumarole vents and areas affected by acid rain and/or spray from the lake, are marked by distinct secondary mineral assocns., with sulfates commonly as prevailing component. The sulfates occur in a wide mineralogical diversity comprising gypsum/anhydrite, various polyhydrated Al-sulfates, alunite-jarosite group minerals, halotrichite-, voltaite- and copiapite-group minerals, epsomite and romerite. Depending on the sub-environment, they are variably assocd. with clay minerals (kaolinite-group and smectite-group), zeolites, SiO2-polymorphs, Fe-(hydro)oxides, Ti-oxides, native sulfur, sulfides, chlorides, fluorides, phosphates and carbonates. Geochem. modeling was performed to identify mechanisms responsible for the formation of the secondary minerals found in the field, and to predict their possible stability under conditions not seen at the surface. The results indicate that the appearance of amorphous silica, hematite, anhydrite/gypsum, pyrite, anatase and kaolinite is relatively insensitive to the degree of acidity of the local aq. system. On the other hand, alunite-jarosite group minerals, elemental sulfur and Al(OH)SO4 only form under acidic conditions (pH < 4). The presence of polyhydrated Mg- and Fe2 +-sulfates is restricted to olivine-bearing rocks exposed to acid rain or brine spray. Modeling suggests that their formation required a repetitive sequence of olivine dissoln. and evapn. in an open system involving limited amts. of fluid. The mineral variety in the crater of Poa´s is remarkably similar to sulfate-bearing assemblages considered to be the product of acid-sulfate alteration on Mars. The analogy suggests that comparable fluid-rock interaction controls operated in Martian volcanic environments.
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    19. 19
      Kalacheva, E.; Taran, Y.; Kotenko, T.; Hattori, K.; Kotenko, L.; Solis-Pichardo, G. Volcano–hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur. J. Volcanol. Geotherm. Res. 2016, 310, 118– 131,  DOI: 10.1016/j.jvolgeores.2015.11.006
      19
      Volcano-hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur
      Kalacheva, Elena; Taran, Yuri; Kotenko, Tatiana; Hattori, Keiko; Kotenko, Leonid; Solis-Pichardo, Gabriela
      Journal of Volcanology and Geothermal Research (2016), 310 (), 118-131CODEN: JVGRDQ; ISSN:0377-0273. (Elsevier B.V.)
      Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ∼ 1000 m above sea level (asl). The fumaroles are characterized by low-temp., HCl- and S-rich gas and numerous hyper-acid pools (pH < 1) without drains. At ∼ 550 m asl, in the Yurieva stream canyon, many hot (up to 87 °C) springs discharge ultra-acidic (pH 1-2) SO4-Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000-2000 t/d of water vapor, < 20 t/d of SO2 and < 5 t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60-80 t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano-hydrothermal system. Oxygen and hydrogen isotopic compn. of water and Cl concn. for Yurieva springs show an excellent pos. correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evapn. of the acidic brine. Temps. calcd. from gas compns. and isotope data are similar, ranging from 150 to 250 °C, which is consistent with the presence of a liq. aquifer below the Ebeko fumarolic fields. Satn. indexes of non-silicate minerals suggest temps. ranging from 150 to 200 °C for Yurieva springs. Trace elements (including REE) and Sr isotope compn. suggest congruent dissoln. of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compns., suggesting complicated boiling-condensation-mixing processes.
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    20. 20
      Rodríguez, A.; Varekamp, J. C.; van Bergen, M. J.; Kading, T. J.; Oonk, P. B. H.; Gammons, C. H.; Gilmore, M. In Copahue Volcano; Tassi, F.; Vaselli, O.; Caselli, A., Eds.; Springer-Verlag: Berlin, Heidelberg, 2015; pp 141– 172 DOI: 10.1007/978-3-662-48005-2 .
      There is no corresponding record for this reference.
    21. 21
      Agusto, M. R.; Caselli, A.; Daga, R.; Varekamp, J.; Trinelli, A.; Dos Santos Afonso, M.; Velez, M. L.; Euillades, P.; Ribeiro Guevara, S. The crater lake of Copahue volcano (Argentina): geochemical and thermal changes between 1995 and 2015. In Geochemistry and Geophysics of Active Volcanic Lakes; Ohba, T., Capaccioni, B., Caudron, C., Eds.; Geological Society, London, Special Publications, 2016; Vol. 437; pp 107– 130.
      There is no corresponding record for this reference.
    22. 22
      Varekamp, J. C.; Ouimette, A. P.; Herman, S. W.; Flynn, K. S.; Bermudez, A.; Delpino, D. Naturally acid waters from Copahue volcano, Argentina. Appl. Geochem. 2009, 24, 208– 220,  DOI: 10.1016/j.apgeochem.2008.11.018
      22
      Naturally acid waters from Copahue volcano, Argentina
      Varekamp, J. C.; Ouimette, A. P.; Herman, S. W.; Flynn, K. S.; Bermudez, A.; Delpino, D.
      Applied Geochemistry (2009), 24 (2), 208-220CODEN: APPGEY; ISSN:0883-2927. (Elsevier Ltd.)
      Volcanic acid sulfate-chloride brines form through absorption of volcanic vapors in shallow reservoirs of meteoric water. Reaction with surrounding volcanic rocks leads to partial neutralization of the fluids and pptn. of secondary minerals. Chem. data of such acid waters from Copahue volcano, Argentina, covering 8 years of observations, show evidence for changes in compn. related to water rock interaction at depth prior to emergence of the fluids at the surface. The chem. compn. changed dramatically during the 2000 eruption of Copahue, with enhanced concns. and fluxes of Mg, Na, Fe and Al, followed in 2001 by rapidly declining concns. and element fluxes. The subsequent 5 years saw more variable element ratios and strong depletions in K and Al. Most incompatible elements are released from the rock matrix stochiometrically, whereas some elements are enriched through vapor input from the magma (As, Pb, Zn). Most fluids have LREE enrichments relative to the rock matrix, but during periods of new magma intrusion the LREE enrichment decreases as does the magnitude of the neg. Eu anomaly in the fluids. These observations are interpreted assuming early dissoln. of plagioclase, olivine and volcanic glass that occurs during intrusion of new magma into the hydrothermal system. The high field strength elements are virtually immobile even in these hot acid fluids, with Nb and Ta more so than Hf and Zr. The mobility of U and Th in these fluids is comparable, at variance with Th behavior in neutral fluids. The local rivers and lakes of Copahue are fertilized by volcanic dissolved P, and most surface waters with pH <3 have high levels of As. The acid fluids from Copahue may be surficial analogs for deep subduction fluids that evolve below zones of arc magma generation as well as for early Mars environments that are thought to have had large acid lakes.
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    23. 23
      Christenson, B. W.; Wood, C. P. Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake, New Zealand. Bull. Volcanol. 1993, 55, 547– 565,  DOI: 10.1007/BF00301808
      There is no corresponding record for this reference.
    24. 24
      Inguaggiato, C.; Censi, P.; Zuddas, P.; Londoño, J. M.; Chacón, Z.; Alzate, D.; Brusca, L.; D’Alessandro, W. Geochemistry of REE, Zr and Hf in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia). Chem. Geol. 2015, 417, 125– 133,  DOI: 10.1016/j.chemgeo.2015.09.025
      24
      Geochemistry of REE, Zr and Hf in a wide range of pH and water composition: The Nevado del Ruiz volcano-hydrothermal system (Colombia)
      Inguaggiato, C.; Censi, P.; Zuddas, P.; Londono, J. M.; Chacon, Z.; Alzate, D.; Brusca, L.; D'Alessandro, W.
      Chemical Geology (2015), 417 (), 125-133CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
      The geochem. behavior of Rare Earth Elements, Zr and Hf was investigated in the thermal waters of Nevado del Ruiz volcano system. A wide range of pH, between 1.0 and 8.8, characterizes these fluids. The acidic waters are sulfate dominated with different Cl/SO4 ratios. The important role of the pH and the ionic complexes for the distribution of REE, Zr and Hf in the aq. phase was evidenced. The pH rules the pptn. of authigenic Fe and Al oxyhydroxides producing changes in REE, Zr, Hf amts. and strong anomalies of Cerium. The pptn. of alunite and jarosite removes LREE from the soln., changing the REE distribution in acidic waters. Y-Ho and Zr-Hf (twin pairs) have a different behavior in strong acidic waters with respect to the water with pH near-neutral. Yttrium and Ho behave as Zr and Hf in waters with pH near neutral-to-neutral, showing super-chondritic ratios. The twin pairs showed to be sensitive to the co-pptn. and/or adsorption onto the surface of authigenic particulate (Fe-, Al-oxyhydroxides), suggesting an enhanced scavenging of Ho and Hf with respect to Y and Zr, leading to superchondritic values. In acidic waters, a different behavior of twin pairs occurs with chondritic Y/Ho ratios and sub-chondritic Zr/Hf ratios. For the first time, Zr and Hf were investigated in natural acidic fluids to understand the behavior of these elements in extreme acidic conditions and different major anion chem. Zr/Hf molar ratio changes from 4.75 to 49.29 in water with pH < 3.6. In strong acidic waters the fractionation of Zr and Hf was recognized as function of major anion contents (Cl and SO4), suggesting the formation of complexes leading to sub-chondritic Zr/Hf molar ratios.
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    25. 25
      Colvin, A.; Rose, W. I.; Varekamp, J. C.; Palma, J. L.; Escobar, D.; Gutierrez, E.; Montalvo, F.; Maclean, A.: Crater lake evolution at Santa Ana Volcano (El Salvador) following the 2005 eruption. In Understanding open-vent volcanism and related hazards: Geological Society of America Special Paper 498; Rose, W. I., Palma, J. L., Delgado Granados, H., Varley, N., Eds.; Geological Society of America, 2013; pp 23– 43.
      There is no corresponding record for this reference.
    26. 26
      Fernández-Remolar, D. C.; Morris, R. V.; Gruener, J. E.; Amils, R.; Knoll, A. H. The Río Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars. Earth Planet. Sci. Lett. 2005, 240, 149– 167,  DOI: 10.1016/j.epsl.2005.09.043
      26
      The Rio Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars
      Fernandez-Remolar, David C.; Morris, Richard V.; Gruener, John E.; Amils, Ricardo; Knoll, Andrew H.
      Earth and Planetary Science Letters (2005), 240 (1), 149-167CODEN: EPSLA2; ISSN:0012-821X. (Elsevier B.V.)
      Exploration by the NASA rover Opportunity has revealed sulfate- and hematite-rich sedimentary rocks exposed in craters and other surface features of Meridiani Planum, Mars. Modern, Holocene, and Plio-Pleistocene deposits of the Rio Tinto, southwestern Spain, provide at least a partial environmental analog to Meridiani Planum rocks, facilitating our understanding of Meridiani mineral pptn. and diagenesis, while informing considerations of martian astrobiol. Oxidn., thought to be biol. mediated, of pyritic ore bodies by groundwaters in the source area of the Rio Tinto generates headwaters enriched in sulfuric acid and ferric iron. Seasonal evapn. of river water drives pptn. of hydronium jarosite and schwertmannite, while (Mg,Al,Fe3+)-copiapite, coquimbite, gypsum, and other sulfate minerals ppt. nearby as efflorescences where locally variable source waters are brought to the surface by capillary action. During the wet season, hydrolysis of sulfate salts results in the pptn. of nanophase goethite. Holocene and Plio-Pleistocene terraces show increasing goethite crystallinity and then replacement of goethite with hematite through time. Hematite in Meridiani spherules also formed during diagenesis, although whether these replaced precursor goethite or pptd. directly from groundwaters is not known. The retention of jarosite and other sol. sulfate salts suggests that water limited the diagenesis of Meridiani rocks. Diverse prokaryotic and eukaryotic microorganisms inhabit acidic and seasonally dry Rio Tinto environments. Org. matter does not persist in Rio Tinto sediments, but biosignatures imparted to sedimentary rocks as macroscopic textures of coated microbial streamers, surface blisters formed by biogenic gas, and microfossils preserved as casts and molds in iron oxides help to shape strategies for astrobiol. investigation of Meridiani outcrops.
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    27. 27
      Nordstrom, D. K.; Alpers, C. N.; Ptacek, K. J.; Blowes, D. W. Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California. Environ. Sci. Technol. 2000, 34, 254– 258,  DOI: 10.1021/es990646v
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      Negative pH and Extremely Acidic Mine Waters from Iron Mountain, California
      Nordstrom, Darrell Kirk; Alpers, Charles N.; Ptacek, Carol J.; Blowes, David W.
      Environmental Science and Technology (2000), 34 (2), 254-258CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Extremely acidic mine waters with pH values as low as -3.6, total dissolved metal concns. as high as 200 g/L, and sulfate concns. as high as 760 g/L, have been encountered underground in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as -4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidn. and concn. by evapn. with minor effects from aq. ferrous iron oxidn. and efflorescent mineral formation.
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    28. 28
      Craig, H.: Geochemistry and origin of the Red Sea brines. In Hot brines and recent heavy metal deposits in the Red Sea, A geochemical and geophysical acoount; Degens, E. T., Ross, D. A., Eds.; Springer: Berlin, Heidelberg, 1969; pp 208– 242.
      There is no corresponding record for this reference.
    29. 29
      Warren, J. K. Danakil Potash, Ethiopia: Beds of Kainite-Carnallite, Part 2 of 4, 2015. SaltWork Consultants Pte Ltd: Kingston Park, Adelaide DOI: 10.13149/RG.2.1.2392.9769 .
      There is no corresponding record for this reference.
    30. 30
      Millero, F. J. The thermodynamics and kinetics of the hydrogen sulfide system in natural waters. Mar. Chem. 1986, 18, 121– 147,  DOI: 10.1016/0304-4203(86)90003-4
      30
      The thermodynamics and kinetics of the hydrogen sulfide system in natural waters
      Millero, Frank J.
      Marine Chemistry (1986), 18 (2-4), 121-47CODEN: MRCHBD; ISSN:0304-4203.
      A review with 81 refs. on the thermodn. and kinetics of the H2S system in natural waters. Equations are derived relating the soly. and ionization of H2S in water and seawater as a function of salinity, temp., and pressure. The limited data available for the interaction of trace metals for HS- are summarized. The kinetics of oxidn. of H2S is examd. as a function of pH, temp., and salinity. The discrepancies in the available data are largely due to the different [O2]/[HS-] ratios used in various studies. Over a limited pH range (6-8) the pseudo first order rate const. for the oxidn. is directly proportional to the activity of HS-.
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    31. 31
      Ohmoto, H.; Lasaga, A. C. Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochim. Cosmochim. Acta 1982, 46, 1727– 1745,  DOI: 10.1016/0016-7037(82)90113-2
      There is no corresponding record for this reference.
    32. 32
      Pittwell, L. R. Some coordination effects in natural waters of Ethiopia. J. Hydrol. 1972, 17, 225– 228,  DOI: 10.1016/0022-1694(72)90005-4
      There is no corresponding record for this reference.
    33. 33
      Rudolph, W.; Brooker, M. H.; Tremaine, P. R. Raman spectroscopic investigation of aqueous FeSO4 in neutral and acidic solutions from 25 to 303 C inner and outer-sphere complexes. J. Solution Chem. 1997, 26, 757– 767,  DOI: 10.1007/BF02767782
      There is no corresponding record for this reference.
    34. 34
      Rull, F.; Sobrón, F. Band profile analysis of the Raman spectra of sulphate ions in aqueous solutions. J. Raman Spectrosc. 1994, 25, 693– 698,  DOI: 10.1002/jrs.1250250732
      There is no corresponding record for this reference.
    35. 35
      Heinrich, C. A.; Seward, T. M. A spectrophotometric study of aqueous iron (II) chloride complexing from 25 to 200°C. Geochim. Cosmochim. Acta 1990, 54, 2207– 2221,  DOI: 10.1016/0016-7037(90)90046-N
      There is no corresponding record for this reference.
    36. 36
      Stefánsson, A.; Seward, T. M. A spectrophotometric study of iron(III) hydrolysis in aqueous solutions to 200 °C. Chem. Geol. 2008, 249, 227– 235,  DOI: 10.1016/j.chemgeo.2007.12.006
      36
      A spectrophotometric study of iron(III) hydrolysis in aqueous solutions to 200°C
      Stefansson, Andri; Seward, Terry M.
      Chemical Geology (2008), 249 (1-2), 227-235CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
      The hydrolysis of iron(III) was studied in acid aq. solns. between 25 and 200° at satd. water vapor pressure by uv-vis spectrophotometry using a high-temp., flow-through gold-lined optical cell. The strong ligand-to-metal charge transitions of iron(III) hydroxo complexes at wavelengths below 400 nm were used to obtain molar absorptivities, ε, and equil. hydrolysis consts. using principal component anal. of the spectra. The total iron(III) concns. ranged from 6.184 × 10-5 to 1.652 × 10-4 mol/kg-1 and the perchloric acid concn. was between 9.31 × 10-4 and 0.398 mol/kg-1. Under these conditions two iron(III) species were identified, Fe3+ and FeOH2+, and the corresponding hydrolysis const. according to the reaction increased from logβ1 = - 2.18 ± 0.01 at 25° to 0.54 ± 0.15 at 200°. The hydrolysis of Fe3+ to form FeOH2+ is predominantly driven by pos. entropy indicating electrostatic interaction between Fe3+ and OH-.
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    37. 37
      Kanno, H.; Hiraishi, J. A Raman study of aqueous solutions of ferric nitrate, ferrous chloride and ferric chloride in the glassy state. J. Raman Spectrosc. 1982, 12, 224– 227,  DOI: 10.1002/jrs.1250120305
      37
      A Raman study of aqueous solutions of ferric nitrate, ferrous chloride and ferric chloride in the glassy state
      Kanno, H.; Hiraishi, J.
      Journal of Raman Spectroscopy (1982), 12 (3), 224-7CODEN: JRSPAF; ISSN:0377-0486.
      Raman spectra were obtained for aq. solns. of Fe(NO3)3, FeCl2, and FeCl3 in the glassy state where R = moles of water/mol of salt = 10-20. The predominant ionic species in these solns. are [Fe(H2O)6]3+, [Fe(H2O)6]2+, and trans-[FeCl2(H2O)4]+, resp. The 2nd dominant ferric species in the glassy FeCl3 soln. (R = 20) seems to be the [FeCl(H2O)5]2+ ion, which is inferred from comparison of the frequencies of the Fe-OH2 stretching Raman bands.
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    38. 38
      Koyama, Y.; Umemoto, Y.; Akamatsu, A.; Uehara, K.; Tanaka, M. Raman spectra of chrolophyll forms. J. Mol. Struct. 1986, 146, 273– 287,  DOI: 10.1016/0022-2860(86)80299-X
      There is no corresponding record for this reference.
    39. 39
      Withnall, R.; Chowdhry, B. Z.; Edwards, S. J. Raman spectra of carotenoids in natural products. Spectrochim. Acta, Part A 2003, 59, 2207– 2212,  DOI: 10.1016/S1386-1425(03)00064-7
      39
      Raman spectra of carotenoids in natural products
      Withnall, Robert; Chowdhry, Babur Z.; Silver, Jack; Edwards, Howell G. M.; de Oliveira, Luiz F. C.
      Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2003), 59A (10), 2207-2212CODEN: SAMCAS; ISSN:1386-1425. (Elsevier Science B.V.)
      Resonance Raman spectra of naturally occurring carotenoids have been obtained from nautilus, periwinkle (Littorina littorea) and clam shells under 514.5 nm excitation and these spectra are compared with the resonance Raman spectra obtained in situ from tomatoes, carrots, red peppers and saffron. The tomatoes, carrots and red peppers gave rise to resonance Raman spectra exhibiting a ν1 band at ∼1520 cm-1, in keeping with its assignment to carotenoids with ∼nine conjugated carbon-carbon double bonds in their main chains, whereas the resonance Raman spectrum of saffron showed a ν1 band at 1537 cm-1 which can be assigned to crocetin, having seven conjugated carbon-carbon double bonds. A correlation between ν1 wavenumber location and effective conjugated chain length has been used to interpret the data obtained from the shells, and the wavenumber position (1522 cm-1) of the ν1 band of the carotenoid in the orange clam shell suggests that it contains nine conjugated double bonds in the main chain. However, the black periwinkle and nautilus shells exhibit ν1 bands at 1504 and 1496 cm-1, resp. On the basis of the correlation between ν1 wavenumber location and effective conjugated chain length, this indicates that they contain carotenoids with longer conjugated chains, the former having ∼11 double bonds and the latter ∼13 or even more. Raman spectra of the nautilus, periwinkle and clam shells also exhibited a strong band at 1085 cm-1 and a doublet with components at 701 and 705 cm-1, which can be assigned to biogenic calcium carbonate in the aragonite crystallog. form.
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    40. 40
      Nugent, P. W.; Shaw, J. A.; Vollmer, M. Colors of thermal pools at Yellowstone National Park. Appl. Opt. 2015, 54, B128– 139,  DOI: 10.1364/AO.54.00B128
      There is no corresponding record for this reference.
    41. 41
      Craig, H. Isotopic variations in meteoric waters. Science 1961, 133, 1702– 1703,  DOI: 10.1126/science.133.3465.1702
      41
      Isotopic variations in meteoric waters
      Craig, Harmon
      Science (Washington, DC, United States) (1961), 133 (), 1702-3CODEN: SCIEAS; ISSN:0036-8075.
      The relation between deuterium and O18 concns. in natural meteoric waters from many parts of the world was detd. with a mass spectrometer. The isotopic enrichments, relative to that of ocean water, display a linear correlation over the entire range for waters that have not undergone excessive evapn.
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    42. 42
      Craig, H.; Geochemistry and Hydrology of Geothermal Waters in the Ethiopian Rift Valley; California, San Diego, 1977.
      There is no corresponding record for this reference.
    43. 43
      Sofer, Z.; Gat, J. The isotope composition of evaporating brines: effect of the isotopic activity ratio in saline solutions. Earth Planet. Sci. Lett. 1975, 26, 179– 186,  DOI: 10.1016/0012-821X(75)90085-0
      43
      Isotope composition of evaporating brines. Effect of the isotopic activity ratio in saline solutions
      Sofer, Z.; Gat, J. R.
      Earth and Planetary Science Letters (1975), 26 (2), 179-86CODEN: EPSLA2; ISSN:0012-821X.
      A math. expression is given for the ratio of the activity coeffs. of deuterated and light water in chloridic saline solns. as a function of the molality of the salt components. Calcns. based on the Craig-Gordon evapn. model show that in the case of evapg. brines, the salt effect on the activity coeffs. of the isotopic water results in deviation of the δ values from a const. slope evapn. line, when plotted on the conventional δ-diagram, of D-concn. vs. 18O activity. Conversion of concn. data to activities eliminates these deviations to a large extent, indicating them to be an artifact of the scales employed for measuring the isotopic abundance. The origin of Dead Sea waters and other natural brines is discussed in the light of these findings.
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    44. 44
      Oerter, E. J.; Singleton, M.; Davisson, M. L. Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions. Geochim. Cosmochim. Acta 2018, 238, 316– 328,  DOI: 10.1016/j.gca.2018.07.009
      44
      Hydrogen and oxygen stable isotope dynamics of hyper-saline and salt-saturated aqueous solutions
      Oerter, Erik J.; Singleton, Michael; Davisson, M. Lee
      Geochimica et Cosmochimica Acta (2018), 238 (), 316-328CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
      An attractive tracer of water-surface interactions in humidity exposure expts. are the stable hydrogen and oxygen isotopes in water vapor, though the conventional method of using salt satd. aq. solns. to control humidity is likely to exert strong effects on the H and O stable isotope compn. of the vapor. The magnitude of these effects are virtually unknown for hyper-saline solns. near salt satn. and beyond. Here we explore the hydrogen and oxygen stable isotope effects of salt-water mixts. from dil. to the salt satn. point and beyond for BaCl2, CaCl2, KCl, K2SO4, MgCl2, NaBr, NaCl, and NaI salts, of which the O isotope results on BaCl2, K2SO4, and NaBr are the first to be reported, and the first for H with BaCl2. We find that vapor humidity levels, and hydrogen and oxygen isotope effects in the vapor generated from dil. solns. to salt-satd. mixts. increase in magnitude by predictable linear trends. In the case of hydrogen isotope salt effects, the cation identity matters, as the effect becomes stronger with decreasing size and increasing charge (increasing ionic potential) in the sequence of: K < Na < Ba < Ca < Mg. For oxygen, the cation effect becomes stronger with increasing ionic potential, in the order of Na < Ba < K < Ca < Mg (irresp. of sign). Hydrogen and oxygen isotope effects in vapor from salt concns. greater than the salt satn. point do not behave as systematically, with some of the salts displaying no increase in isotope effects beyond satn., while others show an increase in isotope effects beyond salt satn., esp. in the case of hydrogen. We make specific recommendations on the salts that exert hydrogen and oxygen isotope effects with the least magnitude and most predictability for humidity levels from 26% to 92%. The use of salt solns. to create vapor at less than 100% and with specific δ2H and δ18O values that differ from that defined by the global meteoric water relationship also seems possible. Hyper-saline surface waters can exist at and above satn., and thus these isotope effects may be present in them, as well as in minerals that form from these waters.
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    45. 45
      Snyder, G.; Poreda, R.; Fehn, U.; Hunt, A. Sources of nitrogen and methane in Central American geothermal settings: Noble gas and 129I evidence for crustal and magmatic volatile components. Geochem., Geophys., Geosyst. 2003, 4, 1– 28,  DOI: 10.1029/2002GC000363
      There is no corresponding record for this reference.
    46. 46
      Kyser, T. K. Stable isotope variations in the mantle. Reviews in Mineralogy and Geochemistry 1986, 16, 141– 164,  DOI: 10.1515/9781501508936-010
      There is no corresponding record for this reference.
    47. 47
      Raven, J. A.; Walker, D. I.; Johnston, A. M.; Handley, L. L. Implications of 13 C natural abundance measurements for photosynthetic performance by marine macrophytes in their natural environment. Mar. Ecol.: Prog. Ser. 1995, 123, 193– 205,  DOI: 10.3354/meps123193
      There is no corresponding record for this reference.
    48. 48
      Sheppard, S. M.; Nielsen, R. L.; Taylor, H. P. Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits. Econ. Geol. Bull. Soc. Econ. Geol. 1971, 66, 515– 542,  DOI: 10.2113/gsecongeo.66.4.515
      48
      Hydrogen and oxygen isotope ratios in minerals from porphyry copper deposits
      Sheppard, Simon M. F.; Nielsen, Richard L.; Taylor, Hugh P., Jr.
      Economic Geology and the Bulletin of the Society of Economic Geologists (1971), 66 (4), 515-42CODEN: ECGLAL; ISSN:0361-0128.
      The 18O/16O, D/H, and 13C/12C values were measured on quartz, K feldspar, biotite, sericite, and clcite from potassic, sericitic, and argillic alteration assemblages and from fresh igneous rocks and veins in 9 North American porphyry Cu and Mo deposits, and in 4 other hydrothermal mineral deposits. Both sericites and biotites preserve their 18O/16O and D/H values, as these isotope ratios are not affected by later low-temp. exchange with local ground waters. A systematic correlation between the D/H ratios of sericites and those of meteoric ground waters indicates the presence of meteoric waters in the hydrothermal fluids involved in sericitization, whereas the restricted range of D/H ratios of biotites indicates the probable dominance of magmatic waters in the fluids assocd. with biotite alteration. The O isotope data for the quartz-K feldspar-biotite-chalcopyrite assemblages also support a high temp. of formation in the presence of waters with δ18O values in the magmatic water range. 13C/12C values of hydrothermal vein carbonates in the porphyry Cu deposits (δ = -2.6 to -5.9) tend to be a little heavier than primary igneous calcites from carbonatites (δ = -5 to -8). It is suggested that heated Na-Ca-Cl brines originally present in the neighboring sedimentary and volcanic sections may be an important component of hydrothermal fluids. The Santa Rita, N. Mex., porphyry Cu deposit isotopic data are discussed in detail.
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    49. 49
      Sheppard, S. M. F. Characterization and Isotopic Variations in natural waters. Reviews in Minerology 1986, 16, 165– 183. DOI: 10.1515/9781501508936-011
      There is no corresponding record for this reference.
    50. 50
      Rye, R. O.; O’Neil, J. R. The 18O content of water in primary fluid inclusions from Providencia, north-central Mexico. Econ. Geol. 1968, 63, 232– 238,  DOI: 10.2113/gsecongeo.63.3.232
      50
      The oxygen-18 content of water in primary fluid inclusions from Providencia, north-central Mexico
      Rye, Robert O.; O'Neil, James R.
      Economic Geology and the Bulletin of the Society of Economic Geologists (1968), 63 (3), 232-8CODEN: ECGLAL; ISSN:0361-0128.
      Water and CO2 extd. from inclusions in sphalerite, calcite, and quartz were analyzed for 18O and 13C. The δ18O of inclusions in sphalerite from 3 different ore bodies ranged 6.2-5.8‰ relative to Standard Mean Ocean Water. These values are lower than the 7-9‰ believed to be typical for pristine magmatic water, and have probably resulted from isotopic exchange between hydrothermal fluids and the cryst. granodiorite through which fluids passed. The δ18O of water in inclusions in calcite and quartz indicate that the inclusions have exchanged 18O with their host during the cooling of the ore deposit, but less than 45% of the exchange that is possible at surface temps. has occurred. The δ13C of CO2 in inclusions in sphalerite and calcite was -7.0 and -11.0‰ relative to PDB. The sphalerite value is consistent with the value previously inferred for δ13C of the hydrothermal fluid, and is believed to be typical of juvenile CO2. The calcite reflects ∼50% of the possible exchange that could occur between inclusion CO2 and the host at the surface.
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    51. 51
      Rothschild, L. J.; Mancinelli, R. L. Life in extreme environments. Nature 2001, 409, 1092– 1101,  DOI: 10.1038/35059215
      51
      Life in extreme environments
      Rothschild, Lynn J.; Mancinelli, Rocco
      Nature (London, United Kingdom) (2001), 409 (6823), 1092-1101CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Each recent report of liq. water existing elsewhere in the Solar System has reverberated through this international press and excited the imagination of humankind. Why Because in the past few decades we have come to realize that where there is liq. water on Earth, virtually no matter what the phys. conditions, there is life. What we previously thought of as insurmountable phys. and chem. barriers to life, we now see as yet another niche harbouring 'extremophiles'. This realization, coupled with new date on the survival of microbes in the space environment and modeling of the potential for transfer of life between celestial bodies, suggests that life could be more common than previously thought. Here we examine critically what it means to be an extremophile, and the implications of this for evolution, biotechnol. and esp. the search for life in the Universe.
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    52. 52
      Harrison, J. P.; Gheeraert, N.; Tsigelnitskiy, D.; Cockell, C. S. The limits for life under multiple extremes. Trends Microbiol. 2013, 21, 204– 212,  DOI: 10.1016/j.tim.2013.01.006
      52
      The limits for life under multiple extremes
      Harrison, Jesse P.; Gheeraert, Nicolas; Tsigelnitskiy, Dmitry; Cockell, Charles S.
      Trends in Microbiology (2013), 21 (4), 204-212CODEN: TRMIEA; ISSN:0966-842X. (Elsevier Ltd.)
      A review. Life on Earth is limited by phys. and chem. extremes that define the 'habitable space' within which it operates. Aside from its requirement for liq. water, no definite limits were established for life under any extreme. Here, we employ growth data published for 67 prokaryotic strains to explore the limitations for microbial life under combined extremes of temp., pH, salt (NaCl) concns., and pressure. Our review reveals a fundamental lack of information on the tolerance of microorganisms to multiple extremes that impedes several areas of science, ranging from environmental and industrial microbiol. to the search for extraterrestrial life.
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    53. 53
      Bekker, A.; Holland, H. D.; Wang, P. L.; Rumble, D., Iii; Stein, H. J.; Hannah, J. L.; Coetzee, L. L.; Beukes, N. J. Dating the rise of atmospheric oxygen. Nature 2004, 427, 117,  DOI: 10.1038/nature02260
      53
      Dating the rise of atmospheric oxygen
      Bekker, A.; Holland, H. D.; Wang, P.-L.; Rumble, D.; Stein, H. J.; Hannah, J. L.; Coetzee, L. L.; Beukes, N. J.
      Nature (London, United Kingdom) (2004), 427 (6970), 117-120CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Several lines of geol. and geochem. evidence indicate that the level of atm. oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels by 2.22 Gyr ago. Here we present evidence that the rise of atm. oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in org.-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic compn. of sulfur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atm. oxygen was present at significant levels (i.e., greater than 10-5 times that of the present atm. level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of heamatitic pisolites and ooelites in the upper Timeball Hill Formation indicate that atm. oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Paleoproterozoic glacial events.
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    54. 54
      Lyons, T. W.; Reinhard, C. T.; Planavsky, N. J. The rise of oxygen in Earth’s early ocean and atmosphere. Nature 2014, 506, 307– 315,  DOI: 10.1038/nature13068
      54
      The rise of oxygen in Earth's early ocean and atmosphere
      Lyons, Timothy W.; Reinhard, Christopher T.; Planavsky, Noah J.
      Nature (London, United Kingdom) (2014), 506 (7488), 307-315CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      The rapid increase of carbon dioxide concn. in Earth's modern atm. is a matter of major concern. But for the atm. of roughly two-and-half billion years ago, interest centers on a different gas: free oxygen (O2) spawned by early biol. prodn. The initial increase of O2 in the atm., its delayed build-up in the ocean, its increase to near-modern levels in the sea and air two billion years later, and its cause-and-effect relationship with life are among the most compelling stories in Earth's history.
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    55. 55
      Halevy, I.; Alesker, M.; Schuster, E. M.; Popovitz-Biro, R.; Feldman, Y. A key role for green rust in the Precambrian oceans and the genesis of iron formations. Nat. Geosci. 2017, 10, 135– 139,  DOI: 10.1038/ngeo2878
      55
      A key role for green rust in the Precambrian oceans and the genesis of iron formations
      Halevy, I.; Alesker, M.; Schuster, E. M.; Popovitz-Biro, R.; Feldman, Y.
      Nature Geoscience (2017), 10 (2), 135-139CODEN: NGAEBU; ISSN:1752-0894. (Nature Publishing Group)
      Iron formations deposited in marine settings during the Precambrian represent large sinks of iron and silica, and have been used to reconstruct environmental conditions at the time of their formation. However, the obsd. mineralogy in iron formations, which consists of iron oxides, silicates, carbonates and sulfides, is generally thought to have arisen from diagenesis of one or more mineral precursors. Ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates. Here we investigate the potential role of green rust, a ferrous-ferric hydroxy salt, in the genesis of iron formations. Our lab. expts. show that green rust readily forms in early seawater-analog solns., as predicted by thermodn. calcns., and that it ages into minerals obsd. in iron formations. Dynamic models of the iron cycle further indicate that green rust would have pptd. near the iron redoxcline, and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments. We suggest, therefore, that the pptn. and transformation of green rust was a key process in the iron cycle, and that the interaction of green rust with various elements should be included in any consideration of Precambrian biogeochem. cycles.
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    56. 56
      Canfield, D. E. The early history of atmospheric oxygen: Homage to Robert M. Garrels. Annu. Rev. Earth Planet. Sci. 2005, 33, 1– 36,  DOI: 10.1146/annurev.earth.33.092203.122711
      56
      The early history of atmospheric oxygen: Homage to Robert M. Garrels
      Canfield, D. E.
      Annual Review of Earth and Planetary Sciences (2005), 33 (), 1-36CODEN: AREPCI; ISSN:0084-6597. (Annual Reviews Inc.)
      A review. This paper reviews the Precambrian history of atm. oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atm. oxygen concns. Also reviewed are the various processes regulating oxygen concns., and several models of Precambrian oxygen evolution are presented. A sparse geol. record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atm. oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXlsV2hsL8%253D&md5=1e7e10bc2bf13bcc222954f65f4144d0
    57. 57
      Klingelhofer, G.; Morris, R. V.; Bernhardt, B.; Schroder, C.; Rodionov, D. S.; de Souza, P. A., Jr.; Yen, A.; Gellert, R.; Evlanov, E. N.; Zubkov, B.; Foh, J.; Bonnes, U.; Kankeleit, E.; Gutlich, P.; Ming, D. W.; Renz, F.; Wdowiak, T.; Squyres, S. W.; Arvidson, R. E. Jarosite and hematite at Meridiani Planum from Opportunity’s Mössbauer Spectrometer. Science 2004, 306, 1740– 1745,  DOI: 10.1126/science.1104653
      57
      Jarosite and hematite at Meridiani Planum from Opportunity's Mossbauer Spectrometer
      Klingelhofer G; Morris R V; Bernhardt B; Schroder C; Rodionov D S; de Souza P A Jr; Yen A; Gellert R; Evlanov E N; Zubkov B; Foh J; Bonnes U; Kankeleit E; Gutlich P; Ming D W; Renz F; Wdowiak T; Squyres S W; Arvidson R E
      Science (New York, N.Y.) (2004), 306 (5702), 1740-5 ISSN:.
      Mossbauer spectra measured by the Opportunity rover revealed four mineralogical components in Meridiani Planum at Eagle crater: jarosite- and hematite-rich outcrop, hematite-rich soil, olivine-bearing basaltic soil, and a pyroxene-bearing basaltic rock (Bounce rock). Spherules, interpreted to be concretions, are hematite-rich and dispersed throughout the outcrop. Hematitic soils both within and outside Eagle crater are dominated by spherules and their fragments. Olivine-bearing basaltic soil is present throughout the region. Bounce rock is probably an impact erratic. Because jarosite is a hydroxide sulfate mineral, its presence at Meridiani Planum is mineralogical evidence for aqueous processes on Mars, probably under acid-sulfate conditions.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BD2cngsVymtQ%253D%253D&md5=7d4dc9d608e3d4d6ba8b288d8995e3bf
    58. 58
      Amils, R.; Fernandez-Remolar, D. Rio Tinto: a geochemical and mineralogical terrestrial analogue of Mars. Life (Basel, Switz.) 2014, 4, 511– 534,  DOI: 10.3390/life4030511
      58
      Rio Tinto: a geochemical and mineralogical terrestrial analogue of mars
      Amils, Ricardo; Fernandez-Remolar, David
      Life (Basel, Switzerland) (2014), 4 (3), 511-534CODEN: LBSIB7; ISSN:2075-1729. (MDPI AG)
      The geomicrobiol. characterization of the water column and sediments of R´io Tinto (Huelva, Southwestern Spain) have proven the importance of the iron and the sulfur cycles, not only in generating the extreme conditions of the habitat (low pH, high concn. of toxic heavy metals), but also in maintaining the high level of microbial diversity detected in the basin. It has been proven that the extreme acidic conditions of R´io Tinto basin are not the product of 5000 years of mining activity in the area, but the consequence of an active underground bioreactor that obtains its energy from the massive sulfidic minerals existing in the Iberian Pyrite Belt. Two drilling projects, MARTE (Mars Astrobiol. Research and Technol. Expt.) (2003-2006) and IPBSL (Iberian Pyrite Belt Subsurface Life Detection) (2011-2015), were developed and carried out to provide evidence of subsurface microbial activity and the potential resources that support these activities. The reduced substrates and the oxidants that drive the system appear to come from the rock matrix. These resources need only groundwater to launch diverse microbial metabs. The similarities between the vast sulfate and iron oxide deposits on Mars and the main sulfide bioleaching products found in the Tinto basin have given R´io Tinto the status of a geochem. and mineralogical Mars terrestrial analog.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvF2qurfK&md5=2592dcca87dc31110a0f0b99d341cbe4
    59. 59
      Bishop, J. L.; Murad, E.; Dyar, M. D. Akaganeite and schwertmannite: Spectral properties and geochemical implications of their possible presence on Mars. Am. Mineral. 2015, 100, 738– 746,  DOI: 10.2138/am-2015-5016
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    TThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.8b00141.

    • 1. Supporting Methods. 1.1. The ferrozine (FZ) method for the Fe speciation in the liquids. 1.2. Isotopic analysis. 1.3. Biological sample preparation for the SEM study. 2. Supporting Results. 2.1. In situ incubations of urea-13C and laboratory experiments. 3. Supporting Figures S-1 to S-3. 3.1. Figure S-1. Aerial photographs and related hydrothermal activity maps of Dallol dome for the 2016 and 2017 field campaigns. 3.2. Figure S-2. Fe species concentration for the spring water (S1) that is dominated by Fe(II) and four successive pools of Dallol (P1–P4) where Fe(III) is gradually increasing over Fe(II). 3.3. Figure S-3. δ13CDIC ‰ of isotopically labeled urea for incubations performed in situ and in the laboratory. 4. Supporting Tables S-1 to S-5. 4.1. Table S-1. Comparative chemical analysis of Dallol brines for the 2016 and 2017 field campaigns 4.2. Table S-2. Oxygen, nitrogen, argon, and carbon stable isotopes and atomic ratios in free gases from springs of Dallol and Black and Yellow Lake waters (sampling 2017). 4.3. Table S-3. Stable isotopes in H2O and DIC (dissolved inorganic carbon) for Dallol hydrothermal brines compared to water from the wider area. 4.4. Table S-4. Oxygen, nitrogen, argon, and carbon stable isotopes and atomic ratios in dissolved gases from Dallol brines and Black and Yellow Lake waters. 4.5. Table S-5. First and second batch of measurements of δ13CDIC‰ of isotopically labeled Urea for incubations performed in selected samples of Dallol pools and sterilized solutions of FeCl2, FeCl3, and (NH4)2S2O8 (PDF)


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