Decoupling of High‐Pressure H2 Production From Serpentinization and Magnetite in Subduction Zones

Serpentinization plays a central role in geological, geochemical, and microbiological processes at various depths and conditions. While the mineralogical and geochemical patterns of serpentinization are known at low‐pressure and temperature conditions characteristic of sub‐seafloor or shallow continental conditions, and favorable conditions for H2 and abiotic CH4 formation at these conditions are also known, equivalent processes happening at greater depths and elevated temperatures in subduction zones are less constrained. Here we present the results of reaction path thermodynamic models simulating irreversible interactions between chemically complex metamorphic aqueous fluids and ultramafic rocks at conditions relevant to three evolutionary stages of subduction, from infancy to maturity, and for three different fluid sources, metabasite, metasediment, and serpentinite. At subduction zone conditions from 300 to 700°C and 1.5–3.0 GPa, serpentinization, H2, and abiotic CH4 production are stronger for high orthopyroxene/olivine ratios, with negligible serpentinization for olivine‐rich starting materials. Furthermore, above brucite dehydration, we found that magnetite production and H2 and CH4 concentrations are decoupled from serpentinization. The degree of serpentinization of the mantle wedge and geophysical fingerprints conventionally attributed to it do not necessarily reflect fluid availability or define potential source regions for deep H2‐CH4‐rich fluids. A new isotope database for complex carbonic fluids allowed computing carbon isotope mass balances for each thermodynamic model. The observed decoupling determines large redox variability, ultimately resulting in carbon isotope signature of abiotic methane within approximately a 15‰ range for different mantle rocks, with important implications on the isotopic diversity of high‐temperature abiotic CH4.