Dionysis I. Foustoukos

Ultramafic‐Hosted Hydrothermal Systems at Mid‐Ocean Ridges: Chemical and Physical Controls on pH, Redox and Carbon Reduction Reactions

Experimental, theoretical and field investigations of hydrothermal alteration processes in ultramafic systems at mid-ocean ridges, indicate that these systems have the capacity to buffer pH at surprisingly low values (pH T,P = 4.9-5.2), which profoundly affects fluid chemistry. Sluggish reaction kinetics of olivine at elevated temperatures and pressures, (e. g., 400°C, 500 bars), together with SiO 2 and Ca dissolution from coexisting pyroxene minerals, enhance the stability of tremolite and talc accounting for the observed acidity. Moreover, oxidation of ferrous silicate components in unstable minerals, especially pyroxenes, generates high H 2(aq) concentrations, which together with the relatively low pH, increase Fe solubility, consistent with the Fe-rich nature of vents fluids issuing from ultramafic-hosted hydrothermal systems at Rainbow and Logatchev at 36°N and 14°N, respectively, on the Mid-Atlantic Ridge. The high dissolved Cu and Ni concentrations, and low H 2 S (aq) of these vent fluids, indicate redox buffering by magnetite-bornite-chalcocite-heazelwoodite (Ni 2 S 3 )-fluid equilibria, as indicated by experimental and theoretical data. Data show that dissolved Cu is particularly sensitive to temperature change, while H 2 S (aq) and Fe are affected less by this, although Fe is highly sensitive to pH and dissolved chloride. Dissolved chloride concentrations observed for both the Rainbow and Logatchev hydrothermal systems depart significantly from seawater and suggest supercritical phase separation in subseafloor reaction zones. The relatively high temperatures required for this, together with the high rates of fluid flow at Rainbow, indicate a magmatic heat source. The most unusual feature of fluids issuing from the Rainbow and Logatchev hydrothermal systems, however, involves high dissolved concentrations of methane and other hydrocarbon species, and detectable carbon monoxide. Experimental data indicate that reducing conditions and mineral catalytic effects may account for this, although the reported CO (aq) at Rainbow is well below predicted levels, suggesting re-equilibration at lower temperatures.

The structure of water-saturated carbonate melts

Abstract The structure of water-saturated Ca- and Mg-bearing carbonate melts under reducing and oxidizing conditions was investigated in a series of hydrothermal anvil cell experiments conducted at 400-1100 °C and 442-2839 MPa. Equilibria were investigated in the calcite-H2O, calcite-CaO-H2O, magnesite- H2O, and magnesite-MgO-H2O systems, with redox conditions controlled by Re/ReO2 and Ti/TiO2 assemblages. Melting relationships and the C-O-H speciation of the coexisting aqueous fluid and melt were assessed in situ by Raman vibrational spectroscopy. Hydrous melting of MgCO3-MgO occurred at ~850 °C, 1.5-2 GPa. In the CaCO3-CaO-H2O system, melt was formed at 600-900 °C and pressures of 0.5-1.5 GPa because of melting-point depression imposed by the presence of CaO. The C-O-H speciation of the carbonate melts and coexisting supercritical aqueous solutions was mainly H2O and CO32-, with traces of CO2(aq) and CH4(aq) in the fluid phase. The melt-fluid H2O partition coefficients attained in the Mg-bearing melt (median 0.5) were higher than in the Ca-bearing melt (median 0.3). Under oxidizing redox conditions, dissolved ReO2 - was present in all phases, underscoring the enhanced solubility of metals in carbonate-bearing melts and carbonatites. In effect, the enhanced solubility of H2O along with the ionic nature of the carbonate melts may promote the solvation of ionic species in the melt structure. From in situ vibrational spectroscopy, the v1-CO32- vibration recorded in the melt spectra suggests the presence of intermolecular interactions between the oxygen of the carbonate ion with water dissolved in the melt. The thermodynamic properties of this water appear to be similar to the supercritical aqueous phase. For example, the estimated enthalpy for the breakage of the hydrogen bonding between water molecules attained values of 6.8 ± 1.5 kcal/mol and 8.4 ± 1.3 kcal/mol in the melt and fluid phase, respectively. The calculated partial molar volume of H2O in the melt (~48 ± 6 cm3/mol) is also comparable to the partial molar volume of supercritical water at similar conditions. Interestingly, this value is considerably greater than published partial molar volume values for H2O in silicate melts (10-12 cm3/mol). The pressure-temperature melting relationships of the CaO-CO2-H2O and MgO-CO2-H2O systems highlight the important role of water and alkaline earth oxides on the hydrous melting of the carbonatebearing subducting oceanic crust. Carbonates present in marine sediments or serpentinized peridotites may melt before complete dehydration at the slab-mantle wedge transition zone, and thus, never reach sub-arc depths. To this end, melting of carbonate minerals at crustal temperatures and pressure can contribute to the volcanic CO2 flux at the arc through melt/fluid interactions.

H/D methane isotopologues dissolved in magmatic fluids: Stable hydrogen isotope fractionations in the Earth’s interior

Abstract A series of hydrothermal diamond-anvil cell experiments have been conducted to evaluate the role of supercritical water on the isotopic equilibrium between H/D methane isotopologues at 600-800 °C and 409-1622 MPa. Raman spectroscopy was deployed to investigate the distribution of H/D isotopic molecules formed during hydrothermal decomposition of Si5C12H36 in H2O-D2O aqueous solutions. To this end, the intensities of the fundamental vibrational C-H and C-D modes of deuteromethanes were employed to determine the thermodynamic properties of isotope exchange reactions between H/D isotopologues and to constrain the methane D/H molar ratios. By adjusting the initial volume ratios of silane/H2O-D2O, reactions in the CH4-D2O-H2O system were monitored for gaseous and supercritical-water phases. Discreet differences between the equilibrium constants, describing the relationship between the CH3D-CH2D2-CHD3-CH4 species dissolved in supercritical water or present as a homogeneous gas phase, are revealed. The bulk D/H methane composition in the liquid-system is also twice that of the D/H molar ratios recorded in the gas-bearing system. Accordingly, condensedphase isotope effects are inferred to play a key role on the evolution of H/D isotopologues, likely induced by differences in the solubility of the isotopic molecules driven by the excess energy/entropy developed during mixing of non-polar species in the H2O-D2O structure. Our experiments show that isotope fractionation effects need to account for the presence of condensed matter (e.g., melts, magmatic fluids), even at conditions at which theoretical models suggest minimal (or nonexistent) isotope exchange, but comparable to those within the Earth’s interior.

Metastable equilibrium in the C-H-O system: Graphite deposition in crustal fluids

Abstract The presence of graphite in natural environments is linked to the redox and thermal conditions of C-H-O fluid/graphite equilibrium in hydrothermal veins and metasomatic contacts. A time-series experimental study was performed to investigate the graphite undersaturated C-H-O system at 600 °C and 1000 MPa, and with ƒO₂ ranging from highly reducing (10−23) to highly oxidizing (104). A nonvolatile intermediate carbon phase exhibiting the Raman spectral features of poorly ordered graphite was formed as the system evolves toward equilibrium as a function of run duration. The thermometric empirical expressions using the G and D bands in the spectra of graphite failed to accurately estimate the experimental temperature. Thus, the existing Raman geothermometers appear inadequate to address graphite formation under conditions of metastable equilibria and to account for kinetic effects such as, for example, the degree of crystallinity. The presence of poorly ordered graphitic carbon at all the redox conditions investigated suggests that the disordered structure of the mineral attains an extensive thermodynamic stability field, and that it may be more readily deposited than crystalline graphite. Metastable graphitic carbon could, therefore, function as a precursor and substrate for the formation of the well-ordered phase. Such metastable graphite may provide an intermediate state that facilitates subduction of carbonaceous material, while imposing constraints on the formation mechanisms and the 13C/12C isotopic systematics of deep seated carbonaceous fluids and minerals such as diamonds.

In-situ measurements of fluorine and chlorine speciation and partitioning between melts and aqueous fluids in the Na2O-Al2O3-SiO2-H2O system

Abstract The effect of pressure and temperature on the structure of silicate melts coexisting with silicasaturated aqueous electrolyte fluids enriched in fluorine or chlorine in the Na2O-Al2O3-SiO2-H2O system has been described. In situ measurements were conducted with the samples at desired temperatures and pressures in a hydrothermal diamond-anvil cell (HDAC) by using microRaman and FTIR spectroscopy techniques. The data were acquired at temperatures and pressures up to 800 °C and 1264 MPa, respectively. In silicate melts, the intensity of the infrared bands assigned to the stretch vibration of OH-groups is smaller than those of coexisting molecular H2O when F and Cl are present in the melt structure. This difference reflects the interaction of F or Cl with H2O in the melts. With decreasing pressure and temperature (P-T) conditions, SiF complexes are favored in the melt over that in coexisting fluid, perhaps because of decreasing silicate concentration in fluids with decreasing temperature and pressure. In these melts, the solubility of Cl, likely in the form of NaCl, increases with decreasing P-T conditions, whereas the abundance of such complexes in coexisting fluids decreases in favor of HCl. Our experimental data were employed to model the ascent of a fluid-saturated magma from the upper mantle to the shallow crust. This modeling offers insights into F and Cl partitioning between and the speciation of F and Cl in melts and magmatic fluids. We suggest that the formation of stable SiF and NaCl complexes and their increasing solubilities in silicate melts during magma ascent may explain the late volcanic degassing of F and Cl compared with the degassing behavior of other volatile species.

Very large differences in intramolecular D-H partitioning in hydrated silicate melts synthesized at upper mantle pressures and temperatures

Abstract Hydrated (with D2O and H2O) sodium tetrasilicate glasses, quenched from melts at 1400 °C and 1.5 GPa, are studied using 1H, 2H, and 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy. Whereas D2O and H2O depolymerize the silicate melt to similar degrees, protium and deuterium intramolecular partitioning between different molecular sites within the glasses is very different and exemplified by a strong preferential association of deuterons to sites with short O-D···O distances. This preference is independent of total water content and D/H ratio. Substantially different intramolecular D-H partitioning is also observed in a glass with a model hydrous basalt composition. Such large differences in isotope partitioning cannot result from classic equilibrium fractionation because of the high synthesis temperature. Potential kinetic isotope effects are excluded via a slow quench experiment. The apparent fractionation is likely governed by density/molar volume isotope effects, where deuterium prefers sites with smaller molar volume. Large differences in intramolecular site partitioning in melts could lead to significant differences in D-H partitioning between water-saturated melt and exsolved aqueous fluid (where D/HW,Melt ≠ D/HW,Fluid) during crystallization of Earth’s magma ocean, potentially controlling the D/H content of the Earth’s oceans.

Deferrisoma palaeochoriense sp. nov., a thermophilic, iron(III)-reducing bacterium from a shallow-water hydrothermal vent in the Mediterranean Sea.

A novel thermophilic, anaerobic, mixotrophic bacterium, designated strain MAG-PB1T, was isolated from a shallow-water hydrothermal vent system in Palaeochori Bay off the coast of the island of Milos, Greece. The cells were Gram-negative, rugose, short rods, approximately 1.0 μm long and 0.5 μm wide. Strain MAG-PB1T grew at 30-70 °C (optimum 60 °C), 0-50 g NaCl l- 1 (optimum 15-20 g l- 1) and pH 5.5-8.0 (optimum pH 6.0). Generation time under optimal conditions was 2.5 h. Optimal growth occurred under chemolithoautotrophic conditions with H2 as the energy source and CO2 as the carbon source. Fe(III), Mn(IV), arsenate and selenate were used as electron acceptors. Peptone, tryptone, Casamino acids, sucrose, yeast extract, d-fructose, α-d-glucose and ( - )-d-arabinose also served as electron donors. No growth occurred in the presence of lactate or formate. The G+C content of the genomic DNA was 66.7 mol%. Phylogenetic analysis of the 16S rRNA gene sequence indicated that this organism is closely related to Deferrisoma camini, the first species of a recently described genus in the Deltaproteobacteria. Based on the 16S rRNA gene phylogenetic analysis and on physiological, biochemical and structural characteristics, the strain was found to represent a novel species, for which the name Deferrisoma palaeochoriense sp. nov. is proposed. The type strain is MAG-PB1T ( = JCM 30394T = DSM 29363T).

A Continuous Culture System for Assessing Microbial Activities in the Piezosphere

ABSTRACT Continuous culture under elevated pressures is an important technique for expanding the exploration of microbial growth and survival in extreme environments associated with the deep biosphere. Here we present a benchtop stirred continuous culture bioreactor capable of withstanding temperatures ranging from 25 to 120°C and pressures as high as 69 MPa. The system is configured to allow the employment of media enriched in dissolved gases, under oxic or anoxic conditions, while permitting periodic sampling of the incubated organisms with minimal physical/chemical disturbance inside the reactor. In a pilot experiment, the fermentative growth of the thermopiezophilic bacterium Marinitoga piezophila was investigated continuously for 382 h at 65°C and at pressures ranging from 0.1 to 40 MPa while the medium flow rate was varied from 2 to 0.025 ml/min. The enhanced growth observed at 30 and 40 MPa and 0.025 ml/min supports the pressure preferences of M. piezophila when grown fermentatively. This assay successfully demonstrates the capabilities of the bioreactor for continuous culturing at a variety of dilution rates, pressures, and temperatures. We anticipate that this technology will accelerate our understanding of the physiological and metabolic status of microorganisms under temperature, pressure, and energy regimes resembling those of the Earths piezosphere.