The oxidation state of metasomatised mantle wedge: insights from hydrate-carbonate-bearing peridotite

In subduction environments the fluid phases released by the subducting plates are vehicles for the slab-to-mantle element transfer, leading to the metasomatism, re-fertilisation and partial melting of the mantle. Occurrences of hydrous minerals coexisting with carbonates and C polypmorphs (e.g. phlogopite + magnesite + graphite/diamond) in mantle wedge peridotites evidence that such fluids are represented by C-O-H solutions, derived by dehydration reactions and decarbonation of the slab. The equilibria involving the volatile elements play an important role in controlling the iron oxidation state of mantle silicates and oxides by redox reactions. Alternatively, Fe3+/Fe2+ equilibria between mantle minerals may buffer the fluid speciation, and therefore oxygen fugacities (fO2). Despite a number of studies have been devoted to determine the redox state of the upper mantle, the fO2 of supra-subduction mantle wedge, used as monitor of its oxidation state, 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. Fe3+/ΣFe in garnet can be measured by the "flank method" electron microprobe analyses [2]. The “flank method” has been calibrated on the JEOL 8200 Superprobe at the Dipartimento di Scienze della Terra (University of Milano) using almandine, andradite and skiagite standards with fixed Fe3+/ΣFe (0, 1 and 0.4 respectively). We have synthesised the end-member skiagite in a multianvil apparatus at P=10 GPa and 1100 °C. The experiments were performed starting from a glass and slag produced in a vertical furnace. The correct Fe3+/ΣFe was achieved by controlling the fO2 of the furnace atmosphere using CO-CO2 gas mixes. 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. The “flank method” measurements indicate that garnet from the Sulu peridotite contains significant amounts of Fe3+/ΣFe (0.05-0.06), while garnet from the Ulten peridotite has Fe3+/ΣFe below the detection limits. For peridotite mineral assemblages fO2 can be evaluated from equilibria involving Fe3+-garnet components, such as: 2 Fe2+3Fe3+2Si3O12 (skiagite) = 4 Fe2+2SiO4 (fayalite) + 2 Fe2+2Si2O6 (ferrosilite) + O2 [1]. Up to date, the lack of thermodynamic data for the Fe3+-garnet component (skiagite), and of an appropriate solid solution model for this phase, limited the applicability of this equilibrium. We therefore modelled non-ideal mixing of Al and Fe3+ on the octahedral site for the almandine-skiagite join by a symmetric regular solution model, combining previous experimental and thermochemical data [3,4]. This enabled us to calculate garnet-peridotite fO2, given the presence of Fe3+ in garnet. The determination of fO2 of metasomatised garnet-peridotites is a powerful tool to estimate the speciation of C-O-H fluids. This permits to speculate on the devolatilisation processes in subduction zones and on the mechanism of C-O-H components transfer from the slab to the mantle wedge. References. [1] G. Gudmundsson, B.J. Wood, Contributions to Mineralogy and Petrology, 119, 56-67, 1995; [2] H. Höfer, G.P. Brey, B. Schulz-Dobrick, R. Oberhänsli, European Journal of Mineralogy, 6, 407-418, 1994; [3] A. B. Woodland, H.S.C. O’Neill, American Mineralogist, 78, 1002-1015, 1993; [4] G. Ottonello, M. Bokreta, P.F. Sciuto, American Mineralogist, 81, 429-447, 1996.