Slab-to-mantle “oxidation” transfer: insights from diamond-bearing majoritic garnets (Western Norway)

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 peridotites 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 infiltration of crustal fluids initiated diamond and majoritic garnet crystallisation [1, 2]. The infiltration of subduction fluids in these rocks at increasing depth promoted crystallisation of microdiamonds in polyphase, fluid-related, inclusions and of majoritic garnet in veins. We studied the peak (stage M3) mineral assemblage garnet + orthopyroxene + olivine ± clinopyroxene ± phlogopite, where garnet turns to 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 results progressively enriched in Fe3+/ΣFe up to 0.15. The fO2 values obtained for the Bardane mantle wedge garnet peridotites 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) [3, 4] to mantle wedge garnet peridotites equilibrated at 4-5 GPa (FMQ – FMQ+2) [5]. Such a trend record the same systematic ΔFMQ decrease with depth as subcratonic xenoliths from South Africa [6], but shifted towards higher fugacities. The determination of oxygen fugacity of the hydrate–carbonate-bearing M3 mineral association enabled to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are H2O-CO2 mixtures, whereby the H2O/CO2 ratio increases with increasing pressure. 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 which could be regarded as carrier of dissolved oxidised components from the subducted slab to the mantle wedge [7, 5]. Such a mechanism opens new possibilities to unravel the redox processes occurring in arc mantle sources and their role in precipitating diamond. References. [1] Spengler, D., Van Roermund, H.L.M., Drury, M.R., Ottolini, L., Mason, P.D., Davies, G.R. (2006): Nature, 440, 913–917; [2] Scambelluri, M., Pettke, T., van Roermund, H.L.M. (2008): Geology, 36, 59–62; [3] Carmichael, I.S.E., Frey, H.M., Lange, R.A., Hall, C.M. (2006): Bull. Volcanol., 68, 407–419; [4] Rohrbach, A., Schuth, S., Ballhaus, C., Münker, C., Matveev, S., Qopoto, C. (2005): Contrib. Mineral. Petrol., 149, 685–698; [5] Malaspina, N., Poli, S., Fumagalli, P. (2009): J. Petrol., 50, 1533–1552; [6] Woodland, A.B. & Koch, M. (2003): Earth Planet. Sci. Lett., 214, 295–310; [7] Hirschmann, M.M., (2009): Science, 325, 545–546.