The oxidation state of metasomatized mantle wedge: Insights from C-O-H-bearing garnet peridotite

Oxygenfugacity(fO2)isanimportantparameterindeterminingthe relative stabilities of phase assemblages.Whereas a number of studies have been devoted to determining the redox state of low-pressure assemblages in the mantle system, the fO2 of the supra-subduction mantle wedge is still poorly known. An essential input for fO2 esti- mates is the determination of the ferric^ferrous iron content of key mantle minerals such as garnet, which can be measured using the 'flank method'technique on an electron microprobe.We selected sam- ples of orogenic peridotites from the ultrahigh-pressure Sulu belt (Eastern China) and from the Ulten Zone (Italian Alps) for detailed case studies; these correspond to slices of metasomatized mantle wedge sampled at different depths. They are characterized by the assemblage phlogopite þ magnesite þamphibole in equilib- rium with olivine, orthopyroxene and Fe3þ-bearing garnet. The 'flank method' measurements indicate that these pyrope-rich garnets contain Fe3þ/?Fe up to 0?12^0?14. For peridotite mineral assem- blages fO2 can be evaluated from equilibria involving the Fe3þ- garnet component skiagite (Fe2þ Fe3þ Si O ) on the basis of 3 2312 Fe3þ^Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3þ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solu- tions of Ca^Fe2þ^Mg^Al^Fe3þ-garnet.This allowed us to calcu- late garnet-peridotite fO2, given the presence of Fe3þ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ þ 2) compared with garnet peri- dotite xenoliths from the sub-cratonic mantle equilibrated at similar pressures. The determination of the oxygen fugacity of these hydrate^carbonate-bearing garnet peridotites allowed us to estimate the speciation of C^O^H metasomatic fluids derived from the sub- ducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential (mu)O2, provides a first step in unravelling the rela- tionship between (mu)O2 and the metasomatic phase assemblage