The role of garnet and pyroxenes in redox processes in the mantle wedge

Redox equilibria in the Earth’s mantle control many chemical and physical processes such as magma genesis, chemical differentiation and fluid-related metasomatism. The whole rock oxidation state is conventionally described in petrology by the oxygen fugacity (fO2). Many studies focussed on the redox state of low pressure assemblages in the mantle system, whereas the fO2 of supra-subduction mantle wedge is still poorly investigated. Orogenic garnet peridotites can be important witnesses of the processes occurring in the deep sub-arc mantle. However, the fO2 determination in most metasomatised garnet peridotites is a demanding task because of the large number of phases that may incorporate both ferric and ferrous iron, and that may show zonations. fO2 in high pressure peridotites is traditionally determined from the Fe2+-Fe3+ content of a garnet in equilibrium with olivine and orthopyroxene, but the Fe3+ partitioning among the peridotite mineral phases is often neglected. Works by [1-3] demonstrate that the increase of Fe3+ in garnet with increasing temperature and pressure is the consequence of the redistribution of Fe3+ from clinopyroxene into the garnet. This implies that the Fe3+ enrichment in garnet is not necessarily indicative of high whole-rock oxygen contents or of the interaction with more oxidised metasomatic agents. We selected a sample of orogenic peridotite from the ultrahigh pressure Sulu belt (Eastern China) corresponding to a slice of metasomatised mantle wedge equilibrated at 5 Gpa and 900 °C. This shows phlogopite + magnesite in equilibrium with olivine, orthopyroxene, clinopyroxene and garnet. The Fe3+/ΣFe of garnet, clinopyroxene and phlogopite was measured by two combined techniques: the "Flank Method" on wavelength dispersive spectra, acquired on garnets with electron microprobe at the University of Milano, and electron energy loss spectroscopy, at the Bayerisches Geoinstitut, employed to analyse clinopyroxene and phlogopite, and to check possible zonations. The measurements indicate that the pyrope-rich garnets are zoned and contain Fe3+/ΣFe up to 0.12–0.14. These results are consistent with the relatively high oxygen fugacities (FMQ to FMQ+2) previously calculated for this sample [4]. Results on clinopyroxene and phlogopite indicate that only clinopyroxene contains some amounts of Fe3+ (Fe3+/ΣFe=0.48–0.51), while phlogopite is below the detection limits. Interestingly, also orthopyroxene contain some Fe3+/ΣFe, up to 0.10. Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning apparently do not follow the same trend described by [1] for a suite of sub-cratonic garnet peridotite xenoliths. The determination of Fe2+/Fe3+ equilibria in mineral assemblages of metasomatised mantle-wedge peridotites therefore represents only the first step in unravelling the relationships between fO2 and phase assemblages in multi-component mantle systems. References: [1] Canil D., O’Neill H.S.C. (1996): J Pet 37, 609–635; [2] Woodland A.B., Koch, M. (2003): Earth Planet Sci Lett 214, 295–310; [3] Woodland A.B. (2009): Lithos, in press; [4] Malaspina N., Poli S., Fumagalli P. (2009): J Petrol 50, 1533–1552.