Tracing the source of oxidizing fluids in subduction zones using iron isotopes in garnet

Abstract

Subduction zones are the primary areas of chemical and mass transfer between the Earth’s surface and the mantle. Dehydration during subduction has been linked to subduction seismicity, arc volcanism, and redox (fO2) changes in the subducting slab and overlying mantle wedge. Despite this, no petrologic record tracing the source of oxidizing fluids from the down going slab has yet been observed. To address this, this study shows a direct record of progressive redox change recorded in zoned garnet crystals from Sifnos and Syros, Greece that grew through the breakdown of the hydrous mineral lawsonite during subduction. Oxygen fugacities (fO2) calculated using garnet-epidote oxybarometry for multiple growth zones within single garnet grains have been compared with stable iron isotope compositions in the same growth zone. These combined measurements reveal that garnet cores grew under oxidized conditions, recording higher fO2 and lower 56Fe values, whereas garnet rims grew under more reduced conditions with lower fO2 and higher 56Fe values. This is consistent with the release of oxidizing fluids into the sub-arc mantle accompanying lawsonite breakdown and dehydration, leaving behind a progressively reduced residual slab mineral assemblage. These coupled fO2 and Fe isotope data show that slab dehydration accompanying lawsonite breakdown plays an important and measureable role in the global redox budget, and provides a mechanism for sub-arc mantle oxidation.