Redox controls on Ni–Fe–PGE mineralization and Re/Os fractionation during serpentinization of abyssal peridotite

Awaruite (Ni2Fe to Ni3Fe) is often used to infer fugacity and redox gradients in hydrothermally altered peridotites. However, discrepant petrological and thermodynamic data suggest that the fO2–fS2 stability field of awaruite is not well constrained. In this study, we assess the thermodynamic properties of awaruite and re-evaluate the Fe–Ni–S systematics of hydrothermally altered peridotites. New experimental data indicate that awaruite is stable at higher fO2 than previously thought, supporting the common occurrence of awaruite in the reaction zone of modern and ancient ultramafic-hosted hydrothermal vent systems. Awaruite is known to catalyze the abiogenic synthesis of methane during active serpentinization, contributing to methanogenesis at modern oceanic hydrothermal systems and potentially on early Earth. The enhanced stability field of awaruite determined here suggests that abiogenic methanogenesis may be active at a broad range of redox conditions. Serpentinized peridotites also contain platinum-group element (PGE)–Re sulfides and metal alloys that can be derived from desulfurization of primary mantle sulfides under low fS2. Thermodynamic calculations suggest that Os will exist as metal and Re as ReS2 in the redox stability field of awaruite. Under a relatively broad range of fS2–fO2 conditions, hydrothermal alteration and desulfurization of primary mantle may produce open system behavior in the Re/Os system and variable Re–PGE ratios and contribute over time to the Os isotopic heterogeneity of the mantle, through subduction and recycling of altered oceanic lithosphere. PGE-metal alloys occur at such low abundances in peridotite that they often are nearly impossible to find. The stability of awaruite at higher fO2 suggests that it can coexist with Os–Ir–Pt metal alloys, making it an important indicator mineral for their presence in hydrothermally altered peridotites.