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

摘要: Low temperature vent fluids (<91 °C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30°N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H2,CH4) while attaining elevated pH values, indicative of serpentization processes that govern water/rock interactions deep oceanic crust. Here, we present a series theoretical models to evaluate extent alteration and assess effect cooling on systematics pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate mineral assemblage composed diopside, brucite chrysotile likely dictates fluid moderate serpentinization (<300 °C), by imposing constraints aCa++/a2H+ ratios activity SiO2. Based Sr abundances 87Sr/86Sr isotope reported estimated mass (w/r = 2–4) consistent with published involving CO2 alkane concentrations. Combining δ18O values (0.7‰) such w/r ratios, allows us bracket subseafloor reaction temperatures vicinity 250 °C. These estimates agreement previous studies supporting extensive conductive heat loss within upflow zones. Experimental peridotite-seawater suggest increases during which then rapidly enhances boron removal solution incorporation into secondary phases, providing an explanation for highly depleted concentrations measured low but alkaline City fluids. Finally, account 11B composition (δ11B ∼25–30‰) relative seawater, isotopic fractionation between tetrahedrally coordinated species BO3-bearing sites (e.g. calcite, brucite) is proposed.