Chemical and isotopic constraints on water/rock interactions at the Lost City hydrothermal field, 30°N Mid-Atlantic Ridge

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.