Metamorphism of peritotites in the mantle wedge above the subduction zone: Hydration of the lithospheric mantle

Abstract

Two areas with different types of hydration (serpentinization), which occurred in two settings distinct in temperatures, pressures, and stresses, are spatially individualized in the ophiolitic ultramafic massifs of the Polar Urals. The high-temperature hydration of ultramafic rocks occurred in the lithosphere of the mantle wedge directly above the subducted slab. The initial conditions of hydration are limited to 1.2–2 GPa and 650–700°C; a stable assemblage of olivine + antigorite + magnetite → amphibole → talc → chlorite was formed at 0.9–1.2 GPa and 550–600°C. The low-temperature mesh lizardite–chrysotile serpentinization occurred in the crustal, near-surface conditions. Both types of hydration were accompanied by release of hydrogen, which participates in abiogenic CH₄ synthesis in the presence of CO₂ dissolved in water.