The geologic record of hydration and dehydration in the subducting slab: Epidote minerals record alteration and metamorphism before and during subduction

The dehydration of oceanic crust in subduction zones is a key control on subducting plate interface rheology and global element and fluid budgets. The “lower” grade metamorphic history of dehydration in subducted metabasalts in warm subduction zones is particularly important as these rocks may carry significant volumes of water into subduction zones, some of which may be released in the forearc at depths where deep slow slip and tremor occur in modern subduction zones, helping to generate local high pore fluid pressures. To explore the geologic record of such dehydration reactions, we synthesize petrologic observations, bulk rock and epidote major- and trace-element geochemistry, Sr-isotope data, and thermodynamic modeling of epidote-amphibolite facies metabasalts in the Catalina Schist of CA, USA. These metabasalts represent exhumed slices of oceanic crust that experienced peak P-T conditions of ~550°C and ~1 GPa and were underplated during Cretaceous subduction beneath North America. We focus on using epidote-group minerals in these rocks as a recorder of hydration and dehydration processes because epidote commonly forms during seafloor hydrothermal alteration and is a predicted reaction product in thermodynamic modeling of metabasalts during prograde subduction. Indeed, the Catalina Schist metabasalts include textural and geochemical evidence of seafloor hydration in the form of interpillow epidosite, epidote trace element patterns, and bulk-rock Sr-isotope values. However, these rocks also include metamorphic epidote porphyroblast and vein-like networks that developed during prograde metamorphism. Based on in-situ epidote trace element analyses, we suggest that metamorphic epidote in these rocks grew from pumpellyite breakdown, in some cases resulting in the development of epidote-rich zones that represent loci of dehydration and possible fluid pathways. We suggest these vein-like zones developed as a result of density changes during this reaction. Using thermodynamic models of the bulk rock composition of these metabasalts, we estimate that this pumpellyite to epidote reaction occurred at ~300°C and 0.5–0.7 GPa. The results of our thermodynamic modeling further suggest that the pumpellyite to epidote reaction resulted in a change in the water content of a hydrated Catalina Schist basalt from 5.5 wt. % H₂O to 2.5 wt. % H₂O, or a release of ~90 kg H₂O per cubic meter of basalt.   This corresponds to a flux of water from a 600 m-thick pile of altered basalts on the order of 10⁴–10⁵ kg m^-2 Myr^-1 in the region experiencing dehydration. In modern subduction zones, a dehydration pulse at these conditions would provide significant volumes of fluid at the updip end of the deep slow slip and tremor source region, and may provide a local source for inferred elevated pore fluid pressures. Our petrologic and geochemical observations paired with thermodynamic modeling provide insight into the metamorphic reactions that may deliver water to the plate interface at the conditions of slow slip, and show that epidote-group minerals are a powerful tools for exploring relatively “low” grade metamorphic processes in subduction zones (e.g., lithologies lacking garnet).