Redox dynamics of subduction revealed by arsenic in serpentinite

Redox dynamics of subduction processes remain poorly constrained owing to the lack of direct geochemical tracers. We studied, using X-ray absorption spectroscopy, the chemical and redox state of arsenic in the Tso Morari serpentinites that are witnesses of the Himalayan subduction. Our measurements reveal remarkably contrasting redox speciation, from arsenide (As – III ) to arsenite (As III ) and arsenate (As V ). Combined with physical-chemical constraints, these data enable reconstruction of the ‘ redox travel ’ of arsenic in the subduction process. Upon early serpentinisation of mantle peridotite, arsenic was scavenged from the fluid and dragged down as insoluble nickel arsenide. Partial deserpentinisation close to the peak metamorphism (550 – 650 °C) resulted in oxidative dissolution of arsenide to aqueous As III and As V and their non-specific intake by antigorite. The As V /As III ratios ( ∼ 0.1 – 10) analysed in the mineral are ∼ 10 4 times higher on average than predicted assuming bulk system thermodynamic equilibrium. These findings reflect a transient out-of-equilibrium release of highly oxidised fluids, with f O 2 reaching ∼ 10 log units above the fayalite-magnetite-quartz buffer (FMQ þ 10). Arsenic in serpentinite is thus a sensitive record of subduction redox dynamics inaccessible when using traditional equilibrium approaches applied to bulk fluid-mineral systems.