Fe3+/Fe2+ EQUILIBRIA IN SUBDUCTION ZONE GARNET PERIDOTITES AS MONITORS OF THE OXIDATION STATE OF THE MANTLE WEDGE

It is widely known that the oxidation state of the mantle strongly influence phase relations, fluids composition and speciation, trace element partitioning, element diffusivity and mechanical behaviour of the mantle. Despite a number of studies heve been devoted to determine the redox state of the upper mantle, the oxygen fugacity (fO2) of supra-subduction mantle wedge, used as monitor of its oxidation state, is still poorly investigated. A major debated topic is addressed to what controls fO2 in the mantle wedge. In such 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. These fluids are represented by compelx C-O-H solutions, derived by the dehydration reactions and decarbonation of the slab. Natural and experimental evidences witness the coexistence of hydrates, carbonates and C polymorphs (e.g. phlogopite + magnesite + graphite/diamond) up to 150 km depth in subduction environments. Fe3+/Fe2+ equilibria among mantle minerals may buffer oxygen fugacities and fluid speciation between C, O and H. Alternatively, equilibria between the volatile elements may control Fe3+/Fe2+ equilibria in mantle silicates and oxides by redox reactions. At present day the possibility to solve such dilemma is limited by the difficulties in determining the redox state of iron in natural mineral assemblages and by the lack of experimental works at high pressure and under controlled oxygen fugacity. An essential input for fO2 estimates may be therefore represented by an accurate determination of the ferric-ferrous iron content of key mantle minerals such as garnet and pyroxene. A method for measuring the Fe2+/Fe3+ ratio of such minerals is the “flank method” with electron microprobe analyses (Höfer et al., 1994). This method is based on the observation that in the FeL X-ray emission spectra, the Lα and Lβ peaks and Lα/Lβ intensity ratios shift with changes in the iron oxidation state. For this reason, Lα/Lβ intensity ratios are measured in correspondence of those flank wavelength positions of both emission lines where the differences between the spectra of pure ferrous or ferric iron-bearing samples are most pronounced. Fe2+/Fe3+ have been measured in mantle mineral assemblages of supra-subduction zone peridotite samples, where hydrates and carbonates coexist in the same assemblage. At this purpose, two cases of study are investigated: (i) metasomatised garnet orthopyroxenite from Maowu, which consist of an older garnet (Grt1) + orthopyroxene (Opx1) + olivine ± clinopyroxene paragenesis overgrown by a new assemblage made of garnet (Grt2) + orthopyroxene (Opx2) ± phlogopite; and (ii) mantle wedge garnet peridotite from the UHP Sulu belt (Eastern China), where magnesite + phlogopite occur in equilibrium with olivine + orthopyroxene + clinopyroxene + garnet. Preliminary data performed in the garnet orthopyroxenite samples indicate that Fe3+/∑Fe is 0.047 for Grt1 and 0.061-0.066 for Grt2. Main goals of this work are: (i) determination of the abundance of ferric iron in various mantle wedge natural samples and (ii) thermodynamic modelling of C-O-H-bearing mantle systems. These data will enable us to compare the buffering capacity of Fe2+/Fe3+ in the upper mantle, relative to the C-O-H fluid speciation, or alternatively to unravel which parameter controls the mantle oxidation state.