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High-pressure experimental data on sound velocities of garnets are used for interpretation of seismological data related to the Earth’s upper mantle and the mantle transition zone. We have carried out a Nuclear Inelastic Scattering study of iron-silicate garnet with skiagite (77 mol%)–iron–majorite composition in a diamond anvil cell up to 56 GPa at room temperature. The determined sound velocities are considerably lower than sound velocities of a number of silicate garnet end-members, such as grossular, pyrope, Mg–majorite, andradite, and almandine. The obtained sound velocities have the following pressure dependencies: V p [km/s] = 7.43(9) + 0.039(4) × P [GPa] and V s [km/s] = 3.56(12) + 0.012(6) × P [GPa]. We estimated sound velocities of pure skiagite and khoharite, and conclude that the presence of the iron–majorite component in skiagite strongly decreases V s . We analysed the influence of Fe3+ on sound velocities of garnet solid solution relevant to the mantle transition zone and consider that it may reduce sound velocities up to 1% relative to compositions with only Fe2+ in the cubic site.

Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia 3 d , a = 11.7511(2) A, V = 1622.69(5) A3, D calc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mossbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X-ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X-ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

Oxygen fugacity (fO2) is an important parameter in determining the relative stabilities of phase assemblages. Whereas a number of studies have been devoted to determining the redox state of low-pressure assemblages in the mantle system, the f O2 of the supra-subduction mantle wedge is still poorly known. An essential input for fO2 estimates is the determination of the ferric-ferrous iron content of key mantle minerals such as garnet, which can be measured using the 'flank method' technique on an electron microprobe. We selected samples of orogenic peridotites from the ultrahigh-pressure Sulu belt (Eastern China) and from the Ulten Zone (Italian Alps) for detailed case studies; these correspond to slices of metasomatized mantle wedge sampled at different depths. They are characterized by the assemblage phlogopite + magnesite +amphibole in equilibrium with olivine, orthopyroxene and Fe3+-bearing garnet. The 'flank method' measurements indicate that these pyrope-rich garnets contain Fe3+/ΣFe up to 0.12-0.14. For peridotite mineral assemblages fO2 can be evaluated from equilibria involving the Fe3+-garnet component skiagite (Fe2+3Fe3+2Si3 O12) on the basis of Fe3+-Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3+ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca-Fe2+-Mg-Al-Fe3+-garnet. This allowed us to calculate garnet-peridotite fO2, given the presence of Fe3+ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ + 2) compared with garnet peridotite xenoliths from the sub-cratonic mantle equilibrated at similar pressures. The determination of the oxygen fugacity of these hydrate-carbonate-bearing garnet peridotites allowed us to estimate the speciation of C-O-H metasomatic fluids derived from the subducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential μO2, provides a first step in unravelling the relationship between μO2 and the metasomatic phase assemblage. © The Author 2009. Published by Oxford University Press. All rights reserved.