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 outflux at
the top of the slab. Previous models of fluid migration indicate that
compaction-pressure gradients induced by the dehydration reactions could
drive updip intraslab fluid flow (Wilson et al., 2014). However, how the
initial hydration in the oceanic mantle prior to subduction impacts the
updip 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 slab-fluid outflux at the slab
surface as shallow as 30–40 km depths. This mechanism is more likely in
young slabs (< ~30 Ma), in which the thickness
of the hydrated mantle in the incoming oceanic mantle that is required
to form the slab-parallel dehydration fronts is relatively small
(< ~20 km) because of its warm condition and
thus a relatively thin antigorite stability zone.