Experiments were conducted to investigate the partitioning of Li, Br, Rb, Cs and B between vapor, brine and halite during subcritical and supercritical phase separation in the NaCl-H2O system (388-550 degrees C, 250-350 bars). Results indicate that Li and Br partition preferentially into the low-salinity vapor fluids, while Rb and Cs become more enriched in the coexisting brines. Under more extreme conditions of pressure and temperature in the two-phase region, especially near the vapor-brine-halite boundary, strong salting-out effects imposed on neutral aqueous species enhance significantly partitioning of all trace elements into the low-salinity fluid. Dissolved boron is strongly affected by this and a particularly strong enrichment into vapors is observed, a trend that can be effectively correlated with changes in reduced density. Exclusion of Li, Br, Rb, Cs and B from halite, when precipitated, further increases the solubility of these species in the coexisting Cl-poor fluid. In general, the lack of distortion in the partitioning behavior of trace elements between vapor, brine and/or halite with the transition from subcritical to supercritical conditions in the NaCl-H2O system precludes the need for special reference to the critical point of seawater when interpreting phase relations in submarine hydrothermal systems. The combination of experimentally determined trace element partitioning data with constraints imposed by mineral solubility provides a means to better understand the origin and evolution of hot spring vent fluids. For example, in Brandon hydrothermal system (21 degrees S EPR) supercritical phase separation and subseafloor mixing appear to be the main heat and mass transport mechanisms fueled by a shallow magmatic intrusion, with boron systematics ruling out major contributions from magmatic degassing processes accompanying the near-seafloor volcanism. (c) 2007 Elsevier Ltd. All rights reserved.

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.