Fe3+ distribution between garnet and pyroxenes in metasomatised mantle wedge garnet peridotites

The determination of the oxidation state of most metasomatised garnet peridotites is a demanding task because a large number of phases incorporate both ferric and ferrous iron (e.g. garnet and clinopyroxene), and may show zonations. Oxygen fugacity in high pressure peridotites is traditionally determined from the Fe2+ and Fe3+ contents of garnet in equilibrium with olivine and orthopyroxene. However, the low Fe3+ contents in orthopyroxene as well as the Fe3+ partitioning into further peridotite mineral phases are often neglected. Previous studies demonstrated that the increase of Fe3+ in garnet with increasing temperature does not depend on the whole-rock Fe2O3, but is rather the consequence of the redistribution of Fe3+ from clinopyroxene into garnet. This implies that the Fe3+ content of all the mineral phases and their possible Fe3+ zonations must be considered to obtain reliable quantifications of the oxidation state of the whole rock. This study presents new measurements of Fe3+ in garnet, olivine, clino- and orthopyroxene of a mantle-derived garnet peridotite from Donghai County, the southeastern end of the Sulu ultrahigh pressure terrane. These rocks correspond to a slice of supra-subduction lithospheric mantle wedge, tectonically emplaced into the crust. They record a multistage metasomatism by an alkali-rich silicate melt at high temperature, and a subsequent influx of a slab-derived incompatible element and silicate-rich fluid during the Triassic UHP metamorphism. We employed two “unconventional” techniques to measure the Fe3+/ΣFe content of mineral phases with high spatial resolution: (i) the Flank Method electron microprobe analyses for garnet, performing for the first time quantitative Fe2O3 map analyses on zoned garnets at the University of Milano, and (ii) the electron energy loss spectroscopy (EELS) for garnet, olivine and pyroxenes, at the Bayerisches Geoinstitut. The results indicate that the pyrope-rich metasomatic garnets present a chemical zoning, with the complementary decrease in Al2O3 from ~26 to ~21 wt%, relative to the increase of Fe2O3 from ~0.8 to ~2.5 wt%. Such a trend is likely related to the Fe3+–Al substitution in the garnet octahedral site, which is sensitive to the garnet oxidation state. Clinopyroxenes are diopsidic in composition, whereas olivine and orthopyroxene have ~92 mol% of forsterite and enstatite, respectively. The EELS measurements show that clinopyroxene contains relatively high Fe3+/ΣFe ratios and Na contents, ranging from 0.48 to 0.51 and from 0.13 to 0.17 a.p.f.u., respectively. Interestingly, also orthopyroxene may contain Fe3+/ΣFe up to 0.10 (±0.05), a percentage comparable to that of garnet, with important consequences in the study of redox processes in mantle rocks and in the application of many geothermometers. Garnet/clinopyroxene and orthopyroxene/clinopyroxene qualitative partitioning indicates a minimum partitioning of Fe3+ from clinopyroxene to garnet. The enrichment in Fe3+ of Ca-clinopyroxene requires the incorporation of a NaFe3+Si2O6 (aegerine) component, particularly in garnet peridotites where the Al content of clinopyroxene is buffered by its coexistence with garnet. The coupled Na-Fe3+ enrichment of our clinopyroxene likely suggests a corresponding enrichment in the whole rock. The Fe3+ substitution mechanism into clinopyroxene as aegerine component could be therefore favoured by the influx of alkali-rich metasomatic fluid phases. This suggests that a possible net bulk oxidation and the redistribution of Fe3+ between garnet and pyroxenes could depend on additional variables besides temperature and pressure. Such mechanisms open new possibilities to unravel the redox processes occurring in the portion of mantle wedge interfacing the subducting slab, which is a key location where the mantle redox reactions likely occur.