We demonstrate an approach to examining the metamorphic history of subducting oceanic crust that can complement records of subduction zone chemical cycling derived from studies of igneous rocks produced at volcanic arcs. By merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high-resolution trace element geochemistry, and application to coesite-bearing mafic eclogites representing subduction to depths beneath arcs, we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models. Specifically, we identify a prograde metamorphic reaction, based solely on the zoning of geochemistry and mineral inclusions within garnet, and infer that these features are a record of the breakdown of coexisting clinozoisite + titanite and probable liberation of trace element--laden fluid from the rock during prograde metamorphism. We are then able to assign a specific depth interval for the reaction through calculation of the P-T dependence of the reaction for these eclogites and comparison with a published P-T trajectory. Because of the robust preservation of records of petrologic and geochemical processes by garnet, this methodology is particularly suited for study of ultrahigh-pressure (UHP) eclogites, in which severe retrograde alteration (generally related to exhumation) commonly obscures prograde history.