The oxidation state of mantle wedge majoritic garnet websterites metasomatised by C-bearing subduction fluids

The majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200 km depth. These samples represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1–M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the episodic/progressive infiltration of crustal fluids at increasing depth enhanced crystallisation of diamond and of majoritic garnet. In these rocks microdiamond occurs as daughter phase in polyphase, fluid-related, inclusions and majoritic garnet is present as vein mineral. We studied the peak (stage M3) mineral assemblage garnet + orthopyroxene + olivine ± clinopyroxene ± phlogopite, where garnet composition becomes majoritic with pressure increase from 3 to 6.5 GPa and 800–1000 °C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond. They witness COH fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO2 in the M3 assemblage starting from Fe3+ analyses in majoritic garnet, which is characterised by the progressive enrichment in Fe3+/ΣFe up to 0.15. The fO2 values obtained for the Bardane mantle wedge garnet websterites are up to −2 log units lower than the fayalite–quartz–magnetite (FMQ) buffer along a trend from arc lavas (FMQ + 1.5–FMQ + 3) to mantle wedge garnet peridotites equilibrated at 4–5 GPa (FMQ–FMQ + 2). The determination of oxygen fugacity of the hydrate–carbonate-bearing M3 mineral association enabled us to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are mixtures of silicates and H2O + CO2, where water is the dominant species of the COH component. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an “oxidised” silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge.