Using Silica Activity to Model Redox-dependent Fluid Compositions in Serpentinites from 100 to 700 °C and from 1 to 20 kbar

Abstract

Serpentinization is a metamorphic process that can stabilize highly reduced hydrogen-rich fluids. Previous measurements of elevated CH4 and H2 concentrations in ultramafic-hosted submarine springs indicate that active serpentinization occurs along mid-ocean ridge systems at seafloor pressures (∼<500 bar) and temperatures (∼<350 °C). Serpentinites also exist at higher pressures in subduction zones; for example, during retrograde hydration of the forearc mantle wedge and during prograde deserpentinization within the subducted slab. However, many studies demonstrating the thermodynamic stability of reduced serpentinite fluids have been limited to terrestrial seafloor conditions. To investigate the redox state of serpentinite fluids at elevated pressures, phase equilibria and fluid compositions were computed for 100–700 °C and 1–20 kbar using aqueous silica activity (aSiO2(aq)) as a governing parameter. Silica-sensitive, redox-buffering assemblages were selected to be consistent with previously proposed reactions: SiO2(aq)–fayalite–magnetite (QFM), SiO2(aq)–Fe-brucite–cronstedtite, SiO2(aq)–Fe-brucite–Fe3+-serpentine, plus the silica-free buffer Fe-brucite–magnetite. Fluid species are limited to simple, zerovalent compounds. For silica-bearing redox reactions, aSiO2(aq) is buffered by coexisting ultramafic mineral assemblages in the system MgO–SiO2–H2O. Silica activity and fO2 are directly correlated, with the most reduced fluids stabilized by the least siliceous assemblages. Silica activity and fO2 increase with pressure, but are more strongly dependent on temperature, leading to greater silica enrichment and more oxidized conditions along shallow, warm subduction paths than along steeper, colder paths. Reduced fluids with mCH4/mCO2 > 1 and fO2 below QFM are present only when serpentine is stable, and are favored along all subduction trajectories except shallow P–T paths at eclogite-grade. Values of mH2 and mCO/mCO2 depend strongly on P and T, but also on the choice of redox buffer, especially whether the Fe-serpentine component is cronstedtite or Fe3+-serpentine. Methane and H2S production are thermodynamically favored throughout the P–T range of the serpentinized forearc mantle and in other settings with similar conditions; for example, deep planetary seafloors. The model offers a generalized technique for estimating the redox state of a fluid-saturated serpentinite at elevated P and T, and yields results consistent with previous petrographic and thermodynamic analyses. High-pressure serpentinization may be an important source of reduced species that could influence prebiotic chemistry, support microbial life in the deep biosphere or in deep planetary oceans, or promote greenhouse warming on early Earth.