Unravelling redox processes in mantle wedge peridotites

Oxygen fugacity (fO2) is an important parameter in determining the relative stabilities of phase assemblages. Despite a number of studies have been devoted to determine the redox state of low pressure assemblages in the mantle system, the fO2 of supra-subduction mantle wedge is still poorly investigated. An essential input for fO2 estimates is represented by the determination of ferric-ferrous iron content of key mantle minerals such as garnet, which can be measured by the "flank method" technique with electron microprobe. As case studies, we selected samples of orogenic peridotites from the ultrahigh pressure Sulu belt (Eastern China) and from the Ulten Zone (Italian Alps) corresponding to slices of metasomatised mantle wedge sampled at different depths. They show the occurrence of phlogopite + magnesite and of 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 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 a non-ideal mixing of Al and Fe3+ on the octahedral site, and a 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 enabled 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÷FMQ+2) compared with garnet peridotite xenoliths from sub-cratonic mantle equilibrated at similar pressure conditions. The determination of oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites enabled us to estimate the speciation of C–O–H metasomatic fluids derived from the subducting slab, which result enriched in CO2. These data might suggest that the relatively high fO2 of these mantle wedge peridotites corresponds to a bulk oxidation due to the influx of slab-derived metasomatic fluids. We demonstrated that the variation of fO2 in multi-component systems is not a simple increasing monotonic function of the oxygen content in the compositional space. High fO2 can be attained by lowering the bulk oxygen proportion in the system, because the chemical potential of oxygen (μO2), and therefore its conventional representation through the fO2 space, has a complex pattern as a function of the variable phase assemblages developed in metasomatised peridotites. The evaluation of fO2 of metasomatised mantle-wedge peridotites, representing the oxygen chemical potential μO2, therefore provides the first step to unravel the relationships between μO2 and the metasomatic phase assemblages in multi-component mantle systems.