Hydrothermal fluids enriched in hydrocarbons of apparent abiotic origin vent from Fe-Ni sulfide bearing chimney structures on the seafloor at slow spreading mid-ocean ridges. Here we show results from a hydrothermal experiment using carbon isotope labeling techniques and mineral analytical data that indicate that pentlandite ((Fe2Ni7)S-8) enhances formation of C-2 and C-3 alkanes, while also contributing to the formation of other more complex hydrocarbons, such as alcohols and carboxylic acids. ToF-SIMS data reveal the existence of isotopically anomalous carbon on the pentlandite surface, and thus, for the first time, provide unambiguous evidence that mineral catalyzed surface reactions play a role in carbon reduction schemes under hydrothermal conditions. We hypothesize that hydroxymethylene (-CHOH) serves as intermediary facilitating formation of more complex organic compounds. The experimental results provide an explanation for organic synthesis in ultramafic-hosted hydrothermal systems on earth, and on other water-enriched planetary bodies as well.

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.