Epidote Records (De)hydration From the Seafloor Through Subduction and Underplating: Implications for Subduction Zone Fluid Budgets and Slow Slip

When, where, and how much fluid is released from subducting oceanic crust control deformation, element cycling, and magmatism in subduction zones. However, the palimpsest of tectonometamorphic processes in the exhumed rock record complicates the relation of geologic observations to geodynamic models of subduction fluid sources and transport. We present textural observations and major and trace element geochemical measurements of epidote combined with phase equilibrium modeling to decipher the pressure‐temperature and dehydration history of metabasalts from the Catalina Schist Epidote Amphibolite Unit (California, USA). We distinguish four generations of epidotes, representing (a) hydrothermal alteration of pillow basalts, (b) prograde epidote formation and fluid release during pumpellyite breakdown at 290–310°C and 0.4–0.7 GPa, (c) equilibration at peak P‐T conditions of 550–600°C and 0.6–0.9 GPa, and (d) retrograde epidote (re)growth during underplating and refrigeration. Geochemical data suggest that basalts were hydrated at the seafloor, and during subduction, metabasalts experienced a large pulse of fluid release during pumpellyite breakdown, recorded by epidote‐rich self‐organizing porosity structures, which helped to drain fluid from the subducting slab. This water was released from the subducting slab at depths corresponding to the base of the seismogenic zone and the shallowest depths at which deep slow slip and tremor occur in modern subduction zones, providing a geologic example of local metamorphic dehydration contributing to the maintenance of high pore fluid pressures at the plate interface. We show that epidote can serve as a recorder of progressive metamorphic processes when careful consideration of textures and mineral chemistry is applied.