FLUIDS FROM THE SLAB: THE HISTORY OF METAMORPHISM AND FLUID FLOW IN SUBDUCTED METABASALTS OF THE CATALINA SCHIST RECORDED BY EPIDOTE MINERALS

Thermodynamic modelling work suggests that dehydration reactions in the subducting slab produce fluids that may create high pore fluid pressures in the environment that hosts deep slow slip and tremor in subduction zones. We explore potential sources of the fluids along the plate interface and how these fluids may have moved through the oceanic crust by targeting metabasalts from the Catalina Schist CA, USA, which was constructed by the underplating of subducted material at amphibolite to lawsonite blueschist facies conditions during the Cretaceous. Using petrography, bulk rock and mineral major and trace element geochemistry, Sr-isotopes, and thermodynamic modelling, we study the record of metamorphic reactions, fluid production, and fluid flow through the lens of epidote growth. Previous thermodynamic modelling studies suggest epidote may be a common product of prograde dehydration reactions along warm subduction geotherms, particularly at the conditions of slow slip and tremor. Here, we analyze epidote with the aim of providing empirical insights into the metamorphic reactions experienced by metabasalts and the fluids they produce. The epidote-amphibolite facies unit of the Catalina Schist represents a coherent section of upper oceanic crust underplated at ~550°C and ~1 GPa. Metabasalts outcrop ~100 m structurally below an ultramafic-metasedimentary mélange unit interpreted to be a paleosubduction interface from ~35 km paleodepth. In these metabasalts, epidote textures and geochemical patterns vary significantly at the outcrop and thin section scale. Preserved pillow basalt textures and epidote trace element compositions record initial seafloor alteration, consistent with bulk-rock Sr-isotopes. However, porphyroblastic epidote and epidote in vein networks have distinct geochemical patterns that likely developed during prograde metamorphism. Variations in trace element patterns among the metamorphic epidotes—especially as indicated in compositional zoning in epidote porphyroblasts in deformed metabasalts—suggest growth of epidote during distinct episodes of fluid flow. Our observations, paired with petrologic modeling of these metabasalts, allow us to explore details of the metamorphic and fluid-flow history of subducting metabasalts and help shed light on the role of metamorphic processes at the conditions of deep slow slip and tremor.