Olivine-Hosted Melt Inclusions Track Progressive Dehydration Reactions in Subducting Slabs Across Volcanic Arcs

The stability and breakdown of mineral phases in subducting slabs control the cycling of trace elements through subduction zones. Stability of key minerals and the partitioning of trace elements between these minerals and liquid phases of interests have been charted by natural sample analysis and experimental constraints. However, systematic study from arc front to far back arc has rarely shown that the expected geochemical variations of the slab liquid are actually recorded by natural samples. Complexities arise by uncertainties on the nature of the slab component (melts, fluids and supercritical liquids), source heterogeneities and transport processes. Using data from olivine-hosted melt inclusions sampled along and across the NE Japan and southern Kurile arcs, we demonstrate that experimentally and thermodynamically constrained phase stabilities in subducted materials indeed control the trace element signatures as predicted by these models and experiments. The main reactions that can be traced across arc are progressive breakdown of light rare earth element-rich accessory phases (e.g. allanite), enhanced dehydration of the lithospheric mantle (serpentine breakdown) and changes in the nature of the slab component. This work elucidates subduction zone elemental cycling in a well-characterized petrogenetic setting and provides important constraints on the interpretation of trace element ratios in arc magmas in terms of the prograde metamorphic reactions within the subducting slab.