SIGNIFICANCE OF HYDROUS ECLOGITE IN OCEANIC SUBDUCTION ZONES (Invited Presentation)

The amount and transport mechanisms and pathways of H2O in subducted slabs strongly influence geodynamic and geochemical processes in subduction zones. Of particular importance are the stability, properties, and composition of hydrous minerals such as lawsonite (Lws). Studies of exhumed oceanic subduction complexes document H2O content of metabasaltic crust and associated metasediments at a range of P-T conditions and show that both Lws-bearing blueschist (bls) and eclogite (ecl) contain considerable structurally-bound H2O. Most exhumed oceanic complexes are structurally-coherent with areas of mélange; both coherent and mélange domains record extensive fluid-rock interaction, including formation of metasomatic rocks and veins, with evidence in mineral composition/zoning for mixing of fluids from different sources (mafic, sedimentary, ultramafic). A survey of ~300 metabasites from 30 oceanic complexes shows that epidote blueschists and eclogites contain 0.1-5 wt% H2O, with more H2O in blueschist than eclogite. Lws metabasites typically have higher H2O contents: 2.5-8.5 wt% (bls), 1.2-7 wt% (ecl), and most contain less H2O than predicted by phase equilibrium modeling of typical MORB. Some Lws eclogite has similar amounts or less H2O than associated blueschist, indicating that the bls-ecl transition does not result in a loss of H2O if there is an external source of fluids, as is common at the slab-mantle interface. In the metabasite dataset, most blueschists and eclogites record Lws-stable conditions, and most Lws eclogites record P-T corresponding to modeled conditions for modern slab surfaces, indicating that exhumed Lws eclogites are good archives of metamorphic and deformation processes in the deep forearc, particularly at the transition from a decoupled to coupled interface. Although sparse in the geologic record, Lws eclogite is common at depth in subduction zones, so study of preserved occurrences provides useful information about H2O content and associated deformation behavior at the deep-forearc slab-mantle interface.