Everett L. Shock

INORGANIC SPECIES IN GEOLOGIC FLUIDS : CORRELATIONS AMONG STANDARD MOLAL THERMODYNAMIC PROPERTIES OF AQUEOUS IONS AND HYDROXIDE COMPLEXES

Correlations among experimentally determined standard partial molal thermodynamic properties of inorganic aqueous species at 25 degrees C and 1 bar allow estimates of these properties for numerous monatomic cations and anions, polyatomic anions, oxyanions, acid oxyanions, neutral oxy-acid species, dissolved gases, and hydroxide complexes of metal cations. Combined with correlations among parameters in the revised Helgeson-Kirkham-Flowers (HKF) equation of state (Shock et al., 1992), these estimates permit predictions of standard partial molal volumes, heat capacities, and entropies, as well as apparent standard partial molal enthalpies and Gibbs free energies of formation to 1000 degrees C and 5 kb for hundreds of inorganic aqueous species of interest in geochemistry. Data and parameters for more than 300 inorganic aqueous species are presented. Close agreement between calculated and experimentally determined equilibrium constants for acid dissociation reactions and cation hydrolysis reactions supports the generality and validity of these predictive methods. These data facilitate the calculation of the speciation of major, minor, and trace elements in hydrothermal and metamorphic fluids throughout most of the crust of the Earth.

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species

The standard partial moial properties of organic aqueous species at high pressures and temperatures can be predicted using an adaptation of a revised equation of state for inorganic aqueous ions and electrolytes ( TANGER and HELGESON, 1988), together with correlations among equation of state parameters (SHOCK and HELGESON, 1988). These correlations include a charge-dependent relation between Born coefficients and the standard partial molal entropies of aqueous species at 25 “C and 1 bar (SHOCK et al., 1989). Thermodynamic calculations indicate that in the liquid phase the standard partial molal volumes ( 8’), heat capacities (C?“,), and entropies (SO), as well as the apparent standard partial molal enthalpies of formation (4A”) of aqueous electrolytes with organic anions maximize with increasing temperature at PSAT* and approach -co at the critical point of H20. In contrast, the corresponding properties of neutral organic aqueous species in the liquid phase minimize with increasing temperature PSAT and approach co at the critical point of H20. Predicted equilibrium constants for alkane solubilities and carboxylic acid dissociation reactions at elevated pressures and temperatures are in close agreement with experimental data reported in the literature, which supports the validity and generality of the equations of state as well as the predictive algorithms used in the calculations. As a consequence, high temperature/ pressure standard partial molal properties, as well as equilibrium constants and other reaction properties, can be predicted for reactions involving a wide variety of organic aqueous species for which little or no experimental data are available at temperatures > 25°C. Present capabilities permit such predictions to be made for hvdrothermal and magmatic conditions at pressures and temperatures as high as 5 kb and 1000°C. d

Prediction of the thermodynamic properties of aqueous metal complexes to 1000°C and 5 kb

A large number of aqueous metal complexes contribute significantly to hydrothermal, metamorphic, and magmatic processes in the crust of the Earth. Nevertheless, relatively few thermodynamic data other than dissociation constants (K) for a few dozen of these complexes have been determined experimentally at elevated temperatures and pressures. The calculations summarized below are intended to supplement these experimental data by providing interim predictions of the thermodynamic properties of supercritical aqueous metal complexes using the revised HKF (Helgeson et al., 1981) equations of state for aqueous species (Tanger and Helgeson, 1988; Shock et al., 1992) and correlations among equations of state parameters and standard partial molal properties at 25°C and 1 bar (Shock and Helgeson, 1988, 1990; Shock et al., 1989). These equations and correlations permit retrieval of the conventional standard partial molal entropies (S0), volumes (V0), and heat capacities (CP0) of aqueous metal complexes at 25°C and 1 bar from published values of log K in the supercritical region and the limited number of experimental dissociation constants available in the literature over relatively short ranges of elevated temperature at PSAT (PSAT and SAT are used in the present communication to refer to pressures corresponding to liquid-vapor equilibrium for the system H2O, except at temperatures <100°C, where they refer to the reference pressure of 1 bar). The standard partial molal properties computed in this way can then be used to generate corresponding values of ΔS0, ΔV0, and ΔCP0 of association, which for similar complexes correlate linearly with S0, V0 and CP0, respectively, of the constituent cations and ligands at 25°C and 1 bar. Generalizing these correlations and combining them with the equations of state permits prediction of the temperature and pressure dependence of log K and other thermodynamic properties of a large number of aqueous metal complexes. As a consequence, it is possible to retrieve values of log K at 25°C and 1 bar from the results of hydrothermal experiments at higher temperatures and pressures or to predict values of log K at hydrothermal conditions when no experimental data are available at temperatures and pressures above 25°C and l bar. Such predictions can be made for temperatures and pressures from 0°C and 1 bar to 1000°C and 5000 bars.

Rare earth elements in hydrothermal systems: Estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures

Abstract Standard partial molal thermodynamic properties including association constants for 246 inorganic aqueous rare earth element (REE) complexes with chloride, fluoride, hydroxide, carbonate, sulfate, bicarbonate, nitrate, and orthophosphate can be calculated at pressures from 1 to 5000 bars and temperatures from 0 to 1000°C, using experimental data from the literature and correlation algorithms. Predicted association constants for REE complexes are used to calculate the speciation of the REEs in simulated and natural fluid compositions over ranges of pH, temperature, and pressure. Our results demonstrate that in a generalized chloride-rich hydrothermal fluid, REE transport may be facilitated by formation of chloride, fluoride, and hydroxide complexes at acidic, neutral, and basic pH conditions, respectively. The HREEs (GdLu) are complexed more strongly by fluoride, and less strongly by chloride, than the LREEs (LaSm), whereas at basic pH conditions HREEs and LREEs strongly associate with hydroxide to an equivalent degree. Estimates of REE speciation in natural hydrothermal solutions that differ in composition and geologic setting demonstrate that different REE-complexes predominate in different environments. It follows that ad hoc assumptions about the identity of complexes which are responsible for REE mobility in a given geologic setting are not necessary.

Geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems

Mixing of hydrothermal fluids and seawater at the ocean floor, combined with slow reaction kinetics for oxidation/reduction reactions, provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis. Thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving S, C, Fe, and Mn compounds during mixing of hydrothermal fluids with seawater. For the vent fluid used in the calculations (EPR 21 degrees N OBS), the model indicates that mixing environments are favorable for oxidation of H2S, CH4, Fe2+ and Mn2+ only below approximately 38 degrees C, with methanogenesis and reduction of sulfate or S degrees favored at higher temperatures, suggesting that environments dominated by mixing provide habitats for mesophilic (but not thermophilic) aerobes and thermophilic (but not mesophilic) anaerobes. A maximum of approximately 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal/kg vent fluid is available from methanotrophy, methanogenesis, oxidation of Fe or Mn, or sulfate reduction. The total potential for chemosynthetic primary production at deep-sea hydrothermal vents globally is estimated to be about 10(13) g biomass per year, which represents approximately 0.02% of the global primary production by photosynthesis in the oceans. Thermophilic methanogens and sulfate- and S degree-reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents, where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34S/32S ratios of vent sulfide minerals. The metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the Earth.

Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. Effective electrostatic radii, dissociation constants and standard partial molal properties to 1000 °C and 5 kbar

Within the framework of the revised HKF (H. C. Helgeson, D. H. Kirkham and G. C. Flowers, Am. J. Sci., 1981, 281, 1249) equations of state (J. C. Tanger IV and H. C. Helgeson, Am. J. Sci., 1988, 288, 19), prediction of the standard partial molal thermodynamic properties of aqueous ions and electrolytes at high pressures and temperatures requires values of the effective electrostatic radii of the ions (re), as well as provision for the temperature and pressure dependence of the relative permittivity of the solvent, H2O. Values of the relative permittivity of H2O, together with the Born functions needed to compute the standard partial molal Gibbs free energy, enthalpy, entropy, heat capacity and volume of solvation were calculated as a function of temperature and density from a modified version of the Uematsu–Franck equation (M. Uematsu and E. U. Franck, J. Phys. Chem. Ref. Data, 1980, 9, 1291). The temperature/pressure dependence of re is described in terms of a solvent function designated by g, which was evaluated in the present study at temperatures and pressures to 1000 °C and 5 kbar by regressing experimental standard partial molal heat capacities and volumes of NaCl reported in the literature together with published dissociation constants for NaClo at supercritical temperatures and pressures using the revised HKF equations of state for aqueous species. The calculated values of re decrease substantially with increasing temperature at constant pressure ⩽2 kbar, and with decreasing pressure at constant temperature 400 °C. The equations and parameters summarized below permit calculation of the standard partial molal properties of aqueous species from the revised HKF equations of state over a much more extensive range of temperature than was previously possible.

Geochemical constraints on the origin of organic compounds in hydrothermal systems

It is proposed that abiotic synthesis of organic compounds occurs in metastable states. These states are permitted by kinetic barriers which inhibit the approach to stable equilibrium in the C-H-O-N system. Evidence for metastable equilibrium among organic compounds in sedimentary basins is reviewed, and further evidence is elucidated from hydrous pyrolysis experiments reported in the literature. This analysis shows that at hydrothermal conditions, organic compounds are formed or destroyed primarily through oxidation/reduction reactions, and that the role of temperature is to lower the kinetic barriers to these reactions. These lines of evidence allow the development of a scenario in which abiotic synthesis can occur at hydrothermal conditions through the reduction of CO2 and N2. This scenario can be tested quantitatively with distribution of species calculations as functions of temperature, pressure, hydrogen fugacity (fH2) and initial composition. One example of such a test is given for an early, sudden outgassing of the Earth, in which CO2, H2O, and N2 are transported from the mantle to the atmosphere by hydrothermal solutions. Activities of metastable aqueous organic species which form as a consequence of this process are evaluated at conditions appropriate for seafloor hydrothermal systems, and are found to maximize at about 200 °C and between the oxidation states set by two mineral assemblages common in the oceanic crust.

Petroleum, oil field waters, and authigenic mineral assemblages Are they in metastable equilibrium in hydrocarbon reservoirs☆

Abstract Although the presence of carboxylic acids and carboxylate anions in oil field waters is commonly attributed to the thermal maturation of kerogen or bacterial degractation of hydrocarbons during water-washing of petroleum in relatively shallow reservoirs, they may have also been produced in deeper reservoirs by the hydrolysis of hydrocarbons in petroleum at the oil-water interface. † To test this hypothesis, calculations were carried out to determine the distribution of species with the minimum Gibbs free energy in overpressured oil field waters in the Texas Gulf Coast assuming metastable equilibrium among calcite albite, and a representative spectrum of organic and inorganic aqueous species at reservoir temperatures and pressures. The cohort of waters chosen for this purpose was restricted to include only those for which analyses reported in the literature list separately analytical concentrations of both organic and inorganic carbon. These values were specified in the Gibbs free energy minimization calculations to constrain the fugacity of oxygen (ƒ O 2(g) ) . ‡ This constraint is predicated on the hypothesis that the oxidation of carboxylic acids to CO2 is rapid in the context of geologic time, but slow in terms of the time span of laboratory studies. The calculations resulted in credible solution pHs and activities of aqueous CO2 (aCO2(aq)). The values of log ƒ O 2(g) generated by the calculations exhibit a remarkably smooth distribution with temperature which is similar to, and within the range of those characteristic of common mineral assemblages. Similar variation with temperature is exhibited by values of log ƒ O 2(g) resulting from calculation of the distribution of species with the minimum Gibbs free energy in oil field waters recovered from the San Joaquin basin of southern California. These observations strongly support the hypothesis that homogeneous equilibrium obtains among carboxylate and carbonate species in oil field waters. To determine the extent to which these species may also be in metastable equilibrium with hydrocarbon species in petroleum at the oil-water interface, representative values of the computed fugacities of oxygen in hydrocarbon reservoirs in the Texas Gulf Coast were used together with corresponding values of aCO2(aq) in the waters, to calculate equilibrium activities of various hydrocarbon species in crude oil. The calculations resulted in reasonable activities of n-alkanes with carbon numbers ≳~6–15, depending on the activity of aqueous CO2. However, it appears that n-alkanes with lower carbon numbers in crude oil cannot achieve heterogeneous metastable equilibrium with oxidized carbon-bearing species in the crust of the Earth. The calculations also indicate that Ca2+, H+, CO2, CH3COOH, CH3COO−, and other aqueous species in oil field waters may be in metastable equilibrium at the oil-water interface with hydrocarbons other than the light paraffins in crude oil, as well as with calcite and other minerals in hydrocarbon reservoirs. § If this is indeed the case, the compositions of formation waters can be used together with Gibbs free energy minimization calculations to guide sequential exploration drilling for hydrocarbon accumulations in sedimentary basins. Both thermodynamic and compositional considerations suggest that the fugacity of oxygen in calcite-bearing reservoirs may be controlled at the oil-water interface by metastable equilibrium states among the heavier hydrocarbons in crude oil and/or calcite and the oxidized carbon-bearing species in the aqueous phase. Irreversible reaction of the light paraffins in petroleum with H2O at the oil-water interface to form lighter paraffins and CO2(aq), CH3COOH(aq), and other oxidized carbon-bearing aqueous species is strongly favored by the large chemical affinities of the reactions. Because these irreversible hydrolytic disproportionation reactions are both exergonic and endothermic, they may be mediated at high temperatures and pressures by hyperthermobarophilic archea or bacteria. ∥ However, the extent to which this occurs at the oil-water interface in any given reservoir may depend on whether or not methane can escape from the system. Although analytical data reported in the literature indicate that maturation of crude oil does not occur to an appreciable degree in static hydrocarbon reservoirs, irreversible hydrolytic disproportionation of the light paraffins in petroleum favors maturation of crude oil in flow channels and reservoirs in young dynamic basins in which fluid flow is extensive and oil, water, and gas are in pervasive contact. It appears that irreversible production of carbonic acid during the hydrolytic disproportionation of the light paraffins in petroleum at the oil-water interface may drive much of the diagenetic process in such basins by lowering the pH of the oil field waters. At near-neutral pHs, the reactions favor precipitation of carbonates, but at lower pH values, they favor carbonate dissolution, albitization of plagioclase, illitization of smectite, and other diagenetic reactions. These observations have far-reaching implications with respect to the development and fate of secondary porosity in hydrocarbon reservoirs.

Organic synthesis during fluid mixing in hydrothermal systems

Hydrothermal circulation can lead to fluid mixing on any planet with liquid water and a source of heat. Aqueous fluids with differing compositions, especially different oxidation states, are likely to be far from thermodynamic equilibrium when they mix, and provide a source of free energy that can drive organic synthesis from CO 2 and H 2 , and/or supply a source of geochemical energy to chemolithoautotrophic organisms. Results are presented that quantify the potential for organic synthesis during unbuffered fluid mixing in present submarine hydrothermal svstems, as well as hypothetical systems that may have existed on the early Earth and Mars. Dissolved hydrogen, present in submarine hydrothermal fluids owing to the high-temperature reduction of H 2 O as seawater reacts with oceanic crustal rocks, provides the reduction potential and the thermodynamic drive for organic synthesis from CO 2 (or bicarbonate) as hydrothermal fluids mix with seawater. The potential for organic synthesis is a strong function of the H 2 content of the hydrothermal fluid, which is, in turn, a function of the prevailing oxidation state controlled by the composition of the rock that hosts the hydrothermal system. Hydrothermal fluids with initial oxidation states at or below those set by the fayalite-magnetite-quartz mineral assemblage show the greatest potential for driving organic synthesis. These calculations show that it is thermodynamically possible for 100% of the carbon in the mixed fluid to be reduced to a mixture of carboxylic acids, alcohols, and ketones in the range 250-50°C as cold seawater mixes with the hydrothermal fluid. As the temperature drops, larger organic molecules are favored, which implies that fluid mixing could drive the geochemical equivalent of a metaholic system. This enormous reduction potential probably drives a large portion of the primary productivity around present seafloor hydrothermal vents and would have been present in hydrothermal systems on the early Earth or Mars. The single largest control on the potential for organic synthesis is the composition of the rock that hosts the hydrothermal system.

Global Occurrence of Archaeal amoA Genes in Terrestrial Hot Springs

ABSTRACT Despite the ubiquity of ammonium in geothermal environments and the thermodynamic favorability of aerobic ammonia oxidation, thermophilic ammonia-oxidizing microorganisms belonging to the crenarchaeota kingdom have only recently been described. In this study, we analyzed microbial mats and surface sediments from 21 hot spring samples (pH 3.4 to 9.0; temperature, 41 to 86°C) from the United States, China, and Russia and obtained 846 putative archaeal ammonia monooxygenase large-subunit (amoA) gene and transcript sequences, representing a total of 41 amoA operational taxonomic units (OTUs) at 2% identity. The amoA gene sequences were highly diverse, yet they clustered within two major clades of archaeal amoA sequences known from water columns, sediments, and soils: clusters A and B. Eighty-four percent (711/846) of the sequences belonged to cluster A, which is typically found in water columns and sediments, whereas 16% (135/846) belonged to cluster B, which is typically found in soils and sediments. Although a few amoA OTUs were present in several geothermal regions, most were specific to a single region. In addition, cluster A amoA genes formed geographic groups, while cluster B sequences did not group geographically. With the exception of only one hot spring, principal-component analysis and UPGMA (unweighted-pair group method using average linkages) based on the UniFrac metric derived from cluster A grouped the springs by location, regardless of temperature or bulk water pH, suggesting that geography may play a role in structuring communities of putative ammonia-oxidizing archaea (AOA). The amoA genes were distinct from those of low-temperature environments; in particular, pair-wise comparisons between hot spring amoA genes and those from sympatric soils showed less than 85% sequence identity, underscoring the distinctness of hot spring archaeal communities from those of the surrounding soil system. Reverse transcription-PCR showed that amoA genes were transcribed in situ in one spring and the transcripts were closely related to the amoA genes amplified from the same spring. Our study demonstrates the global occurrence of putative archaeal amoA genes in a wide variety of terrestrial hot springs and suggests that geography may play an important role in selecting different assemblages of AOA.