The Oxidation State of Metasomatized Mantle Wedge: Insights from C-O-H-bearing Garnet Peridotite

Abstract

Oxygen fugacity (fO2) is an important parameter in determining the 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 estimates 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 samples 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 + magnesite +amphibole in equilibrium with olivine, orthopyroxene and Fe3+-bearing garnet. The ‘flank method’ measurements indicate that these pyrope-rich garnets contain Fe3+/ΣFe up to 0·12–0·14. For peridotite mineral assemblages fO2 can be evaluated from equilibria involving the Fe3+-garnet component skiagite (Fe2+3Fe3+2Si3O12) on the basis of Fe3+–Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3+ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca–Fe2+–Mg–Al–Fe3+-garnet. This allowed us to calculate garnet-peridotite fO2, given the presence of Fe3+ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ + 2) compared with garnet peridotite 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 subducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential μO2, provides a first step in unravelling the relationship between μO2 and the metasomatic phase assemblage.