Hydration State and Updip Fluid Migration in the Slab Mantle

Abstract

Fluid production from dehydration reactions and fluid migration in the subducting slab impact various subduction processes, including intraslab and megathrust earthquakes, episodic slip and tremor, mantle wedge metasomatism, and arc-magma genesis. Quantifying those processes requires a good knowledge of the location and amount of fluid release from the slab and eventual outflux at the top of the slab. Compaction-pressure gradients induced by the dehydration reactions could drive fluid flow in the slab, even in the updip direction, but how the initial hydration in the oceanic mantle prior to subduction impacts the fluid flow has not been investigated. Here, we use a 2-D two-phase flow model to investigate this effect under various initial slab-mantle hydration states and slab thermal conditions, both of which impact the depth extent of the stability of hydrous minerals. We focus on the lateral shift between the site of dehydration reactions and the location of fluid outflux at the top of the slab due to intraslab-updip migration. Our results indicate that major updip fluid pathways form along the antigorite and chlorite dehydration fronts sub-parallel to the slab surface. This, in turn, promotes fluid outflux at the slab surface as shallow as 30–40 km depths. This mechanism is more likely in young slabs (<∼30 Ma) as its warm condition results in a relatively thin (<∼20-km thick) stability zones of hydrous phases in the incoming oceanic mantle, which leads to the formation of the slab-parallel dehydration fronts and updip fluid migration.