Low temperature vent fluids (<91 degrees C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30 degrees N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H-2,CH4) while attaining elevated pH values, indicative of the serpentization processes that govern water/rock interactions deep in the oceanic crust. Here, we present a series of theoretical models to evaluate the extent of hydrothermal alteration and assess the effect of cooling on the systematics of pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate that the mineral assemblage composed of diopside, brucite and chrysotile likely dictates fluid pH at moderate temperature serpentinization processes (<300 degrees C), by imposing constraints on the aCa(++)/a(2)H(+) ratios and the activity of dissolved SiO2. Based on Sr abundances and the Sr-87/Sr-86 isotope ratios of vent fluids reported from Lost City, estimated water/rock mass ratios (w/r = 2-4) are consistent with published models involving dissolved CO2 and alkane concentrations. Combining the reported delta O-18 values of vent fluids (0.77 parts per thousand) with such w/r mass ratios, allows us to bracket subseafloor reaction temperatures in the vicinity of 250 degrees C. These estimates are in agreement with previous theoretical studies supporting extensive conductive heat loss within the upflow zones. Experimental studies on peridotite-seawater alteration suggest that fluid pH increases during cooling which then rapidly enhances boron removal from solution and incorporation into secondary phases, providing an explanation for the highly depleted dissolved boron concentrations measured in the low temperature but alkaline Lost City vent fluids. Finally, to account for the depleted B-11 composition (delta B-11 similar to 25-30 parts per thousand) of vent fluids relative to seawater, isotopic fractionation between tetrahedrally coordinated aqueous boron species with BO3-bearing mineral sites (e.g. in calcite, brucite) is proposed. (C) 2008 Elsevier Ltd. All rights reserved.

The chemical composition of mid-ocean ridge hydrothermal vent fluids is thought to reflect conditions within a deep-seated reaction zone. Although temperature and pressure conditions within this region are key parameters that characterize the subseafloor hydrothermal regime and the cooling of mid-ocean ridges, they are poorly constrained. In this paper, we developed a model in which high-temperature, vapor-type (low-salinity) vent fluid silica (Si) and chlorine (Cl) concentrations can be used to define lines in pressure-temperature space whose intersection is used to estimate conditions at the top of the reaction zone, under the simplifying assumption that Si and Cl reflect a common point of equilibration. We apply this model to various basaltic-hosted mid-ocean ridge sites. Results suggest a minimal variation in inferred temperatures, ranging from 415 to 445 degrees C. This lends support to the fluxibility model in which upwelling hydrothermal plumes rise at temperatures that maximize the energy flux. Quartz precipitation due to reequilibration during upflow tends to lower temperature and pressure estimates and can artificially indicate shallower transition from reaction to upflow zone. However, maximum equilibration pressures are site-dependent and compare well with depth to magma chamber imaged by seismic studies. This suggests that vapors circulate close to magma chambers and is difficult to reconcile with models in which mid-ocean ridge hydrothermal circulation occurs in two layers with a substantial layer of convecting brine. Accordingly, equilibration pressure predicted by our model can also be used to infer the depth of the magma chamber at sites where seismic data are not available but where vapor-like fluids have been collected and analyzed.

Despite significant advances in the understanding of hydrothermal processes at mid-ocean ridges, the linkage between focused flow and diffusively flowing vent fluids has remained elusive. Here, we report the distribution of dissolved carbon species in fluids issuing from associated low-, moderate-, and high-temperature vent systems, collected in 2005 at the Main Endeavour Field (MEF), Juan de Fuca Ridge. Excess CO(aq) measured in the moderate- temperature diffuse flow fluids indicates redox disequilibria between CO(aq) and CO2(aq), suggesting hydrothermal circulation of short residence time in near-seafloor mixing zones. According to geochemical modeling, the chemical variability of MEF fluids is consistent with a combination of conductive cooling, reaction transport, and subsequent seawater mixing of a single source fluid. The similar distribution of C-1-C-3 alkanes observed in diffuse and focused flow fluids likely indicates a minimal fingerprint of biological and methanogenic metabolism on organic fluxes, consistent with a possible short-lived hydrothermal near-seafloor circulation. Accordingly, dissolved carbon species in low- temperature vent fluids can serve as geochemical proxies to distinguish the extent of compositional change in subsurface microbial habitats in the MEF hydrothermal fluids and elsewhere. Part of this reflects conditions unique to the near-surface, diffuse flow environment, but the flux of components from deeper-seated hydrothermal processes is important as well. In comparison with earlier measurements (1999 and 2000) of high-temperature vent fluids at MEF, the 2005 data set exhibits lower Li/Cl ratios and H-2(aq) concentrations indicative of higher fluid/rock mass ratios and more oxidizing and/or lower-temperature conditions, respectively.

Hydrothermal experiments were conducted to evaluate the kinetics of H-2(aq) oxidation in the homogeneous H-2-O-2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 degrees C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H-2(aq) and O-2(aq) are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H-2(aq)-O-2(aq) equilibrium become slower with decreasing temperature, allowing excess H-2(aq) to remain in solution. Sluggish reaction rates of H-2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H-2(aq) and O-2(aq), even at temperatures lower than 40 degrees C due to H-2(aq) persistence in the seawater/vent fluid mixtures. For these H-2-O-2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H-2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.

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 degrees C and 1000 MPa, and with f(o2) ranging from highly reducing (10(-23)) to highly oxidizing (10(4)). 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 C-13/C-12 isotopic systematics of deep seated carbonaceous fluids and minerals such as diamonds.

In order to evaluate the oxidation effect of dissolved hydrogen peroxide and the catalytic role of iron oxides on the kinetics of formic acid decarboxylation, a series of flow-through hydrothermal experiments was conducted at temperatures ranging from 80 to 150 degrees C and pressures of 172-241 bar. delta C-13 composition of residual HCOOH(aq) was also monitored to examine kinetic isotope effects associated with oxidation processes. Our results reveal that decomposition of H2O2(aq) in presence of magnetite follows pseudo first order kinetics, highly enhanced relative to the homogeneous H2O2(aq)-HCOOOH(aq)-H2O system, which possibly reflect synthesis of hydroxyl radicals ((OH)-O-center dot) through Fenton processes. The kinetic rate constants of HCOOH(aq) decarboxylation to CO2(aq) are also elevated relative to those previously measured in H2O2(aq) free experiments. However, reaction kinetics are slightly slower in the case of H2O2(aq) aqueous solutions coexisting with magnetite than in the absence of mineral phases. This behavior is attributed to the possible formation of Fe-bearing hydroxyl formate aqueous species that could serve as stable transition states leading to a decrease in the activation entropy of formic acid decomposition.

A series of experiments has been conducted in the H-2-D-2-D2O-H2O-Ti-TiO2 system at temperatures ranging from 300 to 800 degrees C and pressures between similar to 0.3 and 1.3 GPa in a hydrothermal diamond anvil cell, utilizing Raman spectroscopy as a quantitative tool to explore the relative distribution of hydrogen and deuterium isotopologues of the H-2 and H2O in supercritical fluids. In detail, H2O-D2O solutions (1: 1) were reacted with Ti metal (3-9 h) in the diamond cell, leading to formation of H-2, D-2, HD, and HDO species through Ti oxidation and H-D isotope exchange reactions. Experimental results obtained in situ and at ambient conditions on quenched samples indicate significant differences from the theoretical estimates of the equilibrium thermodynamic properties of the H-D exchange reactions. In fact, the estimated enthalpy for the H-2(aq)-D-2(aq) disproportionation reaction (Delta H-rxn) is about -3.4 kcal/mol, which differs greatly from the +0.16 kcal/mol predicted for the exchange reaction in the gas phase by statistical mechanics models.

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 degrees C and 409-1622 MPa. Raman spectroscopy was deployed to investigate the distribution of HID 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 HID 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, condensed-phase 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.