Fluid-induced redox processes at the slab-mantle interface: insights from ultrahigh-pressure garnet peridotites

The slab-to-mantle volatile transfer is related to the fluid speciation, which in turn is a function of fO2, in a system buffered by equilibria involving redox-sensitive elements. The redox processes taking place in the portion of mantle wedge on top of the subducting slab is poorly investigated and the oxidising power of fluids is still unknown. A case study is represented by grt-orthopyroxenites and peridotites from Dabie-Shan (China) hosted by grt-coe gneisses which contain an association of opx + grt ± cpx ± ol, formed at the expenses of a previous grt-peridotite. Grt includes primary polyphase inclusions corresponding to a solute-rich aqueous fluid enriched in LILE and LREE. Orthopyroxenites represent metasomatic layers produced after the reaction of mantle peridotites with a Si-saturated fluid phase sourced by the associated crustal rocks. The trace element pattern of this fluid shows a peculiar LILE signature which is recorded by subduction grt-peridotites from Sulu (China). These samples show porphyroclastic grt, cpx and phl, and a younger paragenesis of fine-grained ol + cpx + opx + phl ± magnesite equilibrated with neoblastic grt. We measured the Fe3+ distribution of the major phases of orthopyroxenite and peridotite with Flank Method electron probe microanalyses and EELS. Pyrope-rich metasomatic grt presents a complementary decrease in Al2O3, relative to the increase of Fe2O3. Diopsidic cpx contains Fe3+/ΣFe up to 0.51 and high Na, requiring the incorporation of an aegirine component. The coupled Na-Fe3+ enrichment in cpx suggests a corresponding enrichment in the whole rock and could be favoured by the influx of alkali-rich metasomatic fluid phases. To investigate the role of deep fluids in the redox processes of the suprasubduction mantle we also measured the Fe3+ in the microprecipitates of polyphase inclusions using EELS on a TEM. The solute content of slab fluids may contain high Fe3+ concentrations and surprisingly inclusion phases such as phl may contain up to 0.70 of Fe3+/ΣFe. If net bulk rock oxidation can be demonstrated, silicate-alkali-rich C-bearing fluids could be regarded as potential carriers of oxidising components to the suprasubduction mantle.