ИСТИНА

Slab/mantle interface is an intriguing geological environment where thermal and oxygen fugacity gradients are superposed on contrasting lithologies. Although it is widely accepted that metamorphic fluids released from the slabs play a crucial role in subduction zones, processes at slab/mantle interface remain poorly studied, so far. We report results of sandwich experiments with analogues of oceanic crust and mantle rocks carried out using piston-cylinder apparatus at 750-850oC and 2.9 GPa corresponding to the Р-Т conditions of the "hot" modern or ancient subduction zones. Powders of carbonate-bearing blueschist or garnet amphibolite in the lower part of the capsules served as a source for COH fluid. The fluid bearing dissolved components (and dacitic melt in the HT runs) migrated upward and reacted with olivine or clinopyroxene + orthopyroxene powders. Experiments showed a contrasting behavior of alkalis within different mantle lithologies. Pervasive fluid (melt) flux into the olivine layer has produced Na-free reaction zone consisting predominantly of orthopyroxene with some magnesite±phologopite at the metabasite-olivine interface. Local formation of orthopyroxene + garnet ± magnesite ± chlorite ± phlogopite occurs in the olivine layer above the reaction zone. Similar fluid (melt) flux into the two-pyroxene layer lead to the formation of Opx- and K-free reaction layer consisting of omphacite+coesite+magnesite with local development of garnet and omphacite above it. Thus, subduction zone fluids (melts) in peridotites are likely enriched in sodium in contrast to K-rich fluids migrating through pyroxenites. Fluid controlled replacement of orthopyroxene by omphacite+coesite (quartz) assemblage may take place not only in pyroxenites but also in mature orthopyroxene-rich reaction zone at metabasite-peridotite (i.e. slab-mantle) interface.