The recycling of oceanic crust into the mantle through subduction is ultimately responsible for the formation of the Earth's continental crust through arc magmatism. The origin of these magmatic processes is thought to be the melting of mantle rocks under volcanic arcs triggered by the release of fluids from the subducting oceanic slab. The composition of the fluids ultimately responsible for this magmatism is the subject of much research, including the study of the metamorphic byproducts of subduction that are occasionally returned to the surface. These garnet-bearing metamorphic rocks (eclogite) are the only samples of material that have been subducted to great depth, but are available for study. Although eclogite can preserve a record of fluids from subduction zones, it is difficult to determine whether the fluid record preserved in these rocks comes from deep in the subduction zone, or is a result of their exhumation. The research funded by this grant will tie the record of fluids preserved in the oxygen isotope composition of garnets that grew during subduction with the pressure and temperature conditions recorded by the chemical composition of the garnets, generating an integrated tectonic and fluid record for these rocks.
Chemically zoned garnets and mineral inclusions entrapped during their growth have been shown to preserve pressure and temperature information from both the subduction of ocean crust and exhumation of eclogite. Slow intragranular diffusion of oxygen in garnet suggests that these garnets will preserve the δ18O of fluids in equilibrium with the garnet along these paths as well. New advances in ion microprobe technique now allow the δ18O of garnet to be measured in situ with high precision. By integrating in situ δ18O analysis of garnet with existing and new quantitative prograde and retrograde P-T-fO2 information and texturally guided geochronology, this study will generate the first detailed P-T-t-f paths for eclogite and related rocks formed in subduction zones. These histories will allow samples of eclogite to serve as probes into the tectonic and fluid conditions of the fore-arc and subduction channel environment of fossil subduction zones. The integrated nature of these data will tie information on fluids directly to the tectonic evolution of these rocks, and allow them to be used to test a number of hypotheses regarding the nature and timing of the fluid record preserved by exhumed subduction-zone metamorphic rocks. Comparison of data sets between rocks with sedimentary and igneous protoliths found in mélange zones and in coherent fault bounded blocks will offer insight into the timing of their juxtaposition, and will allow existing geochemical studies to be tied to a common tectonic and thermal framework.
