Metasomatism During High-Pressure Metamorphism: Eclogites and Blueschist-Facies Rocks

Abstract

AbstractHigh-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks, which represent parts of ancient subducted slabs, are the obvious target for obtaining constraints on the P-T conditions, composition, and type of slab-derived fluids responsible for metasomatism in subduction zone environments since they are the only direct witnesses of such processes. Usually, solute-poor, aqueous fluids, derived by prograde HP metamorphic devolatilisation reactions in subducted slabs, are made responsible for the trace element transfer between the slab and mantle wedge, both of which may have been metasomatised (modification of bulk rock composition) in due process. At deeper levels in subduction zones at UHP conditions, dense, solute-rich transitional fluids (intermediate between hydrous silicate melts and aqueous fluids), or hydrous silicate melts are thought to become more important with regards to element mobilization and transport. Nonetheless, when considering the petrological and fluid inclusion evidence, it seems reasonable to generally assume that solute-poor aqueous fluids (solute <~30 wt.% of fluid) mainly dominate the fluid budget during HP metamorphism of oceanic and continental crust in subduction zones (70–40 km depth). This is supported by experimental data favoring an aqueous fluid under HP conditions in cold subduction zones. These fluids are regarded by some researchers as being relatively inefficient in mobilizing and transporting elements, as, for instance, has been suggested by experimentally derived fluid-mineral partitioning data. This is consistent with some geochemical studies of HP veins, which suggest a locally highly-restricted fluid flow and thus only localized (small-scale) diffusive element transport. Similar conclusions of only short-distance fluid-mediated mass transport have also been reached by some stable isotope studies in various HP terrains and many fluid inclusion studies. However, significant trace element changes have been reported for metasomatic depletion haloes surrounding eclogite-facies dehydration veins, which constitute major conduits of channelized high fluid flow during prograde HP-metamorphism or for zones of high fluid-rock ratios and fluid flow. Consequently, understanding of the metasomatic processes leading to the mobilization and redistribution of major and trace elements within subduction zone rocks is crucial for (quantitatively) evaluating bulk compositional changes, which occurred under HP and UHP conditions.KeywordsFluid InclusionSubduction ZoneOceanic CrustAqueous FluidHigh Field Strength ElementThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.