Variation in sub-arc mantle oxygen fugacity during partial melting recorded in refractory peridotite xenoliths from the West Bismarck Arc

It is debatable whether oxygen fugacity (fO(2)), the usual measure of the oxidation state of a system, can vary during partial melting in the Earth's mantle or not. Notably, samples of mantle peridotite recovered from lavas and pyroclastic deposits in island arcs are mostly more oxidized than those from other tectonic settings. However, the petrological history of sub-arc mantle peridotites, in particular the respective extents to which partial melting and post-melting metasomatism have controlled their fO(2) record, are elusive. It has remained unclear whether the oxidized peridotites have preserved the oxidation state of a melt-depleted, residual mantle wedge or not. Here we report Mossbauer spectroscopy and EPMA measurements of Fe valence states in spinel (Fe3+/Sigma Fe-spinel where Sigma Fe refers to Fe3+ + Fe2+) in a suite of markedly unaltered, sub-arc mantle-derived, harzburgite and dunite xenoliths from the active Ritter volcano (West Bismarck Arc, Papua New Guinea). These rocks, together with similar suites from the Kamchatka and Izu-Bonin arcs, have recently been interpreted to be residues of partial melting in the mantle wedge. The Fe3+/Sigma Fe-spinel decreases from 0.27 +/- 0.04 to 0.11 +/- 0.01 with increasing degrees of melt depletion in the West Bismarck sample suite, as monitored by decreasing Al2O3 (from 0.72 to 0.29 wt%) and modal percentage of orthopyroxene (from similar to 28 to similar to 7 wt%) in bulk rocks. Importantly, Fe3+/Sigma Fe-spinel in the most melt-depleted, orthopyroxene-poor residual samples are significantly lower (down to 0.11 +/- 0.01) than those in melt-percolated harzburgite (0.29 +/- 0.04) and dunite melt channel-cumulates (0.20 +/- 0.01) found at the same sampling sites. The calculated fO(2) in West Bismarck residual samples decreases from + 1.7 +/- 0.2 to -0.5 +/- 0.2 log units relative to the synthetic fayalite-magnetite-quartz redox buffer (Delta logfO(2)(FMQ)) with Al2O3 and orthopyroxene contents. The upper-end Delta logfO(2) for the least meltdepleted, orthopyroxene-rich residual samples are consistent with those for sub-arc mantle harzburgite xenoliths from the Kamchatka and Izu-Bonin arcs recording similar melting degrees, but also those for more fertile lherzolite and harzburgite rocks from the northeastern Japan Arc. In turn, the most melt-depleted, orthopyroxene-poor residual samples have Delta logfO(2) similar to the upper bound recorded in abyssal peridotites. Taking literature data into consideration, the fO(2) spectrum recorded by the West Bismarck sub-arc mantle peridotite suite is modelled here by a two-stage partial melting process. The first-stage oxidation state is near-buffered from lherzolite to orthopyroxene-rich harzburgite by fluxed-melting with volatile-rich, slab-derived components at ca. FMQ + 0.5 to FMQ + 1.5 during the generation of high-partial melting degree, picrite-boninite-andesite oxidized liquids. The second-stage oxidation state is un-buffered during re-melting of residual harzburgite accompanying generation of low-to moderate-degree partial melts such as low-Ca boninite; these magmas preserve more variable fO(2) extending to lower values (FMQ and below) owing to the progressive removal of Fe3+ from their sources with increasing melting degree. The second-stage melting event likely occurs during adiabatic decompression of residual spinel harzburgite to the uppermost mantle wedge. The data in this study support the general hypothesis that Fe3+/Sigma Fe-spinel and fO(2) of residual peridotite (and of the melts formed at equilibrium) can vary during partial melting in the Earth's mantle. These results further provide direct source evidence for the controls of various subduction zone melting processes on the oxidation state of different types of primary arc melts. Melt depletion of mantle wedge sources can result in a progressive decrease in the fO(2) of liquids subsequently extracted from these sources, but only in the absence of oxidized, Si- and volatile-rich components. These components are presumably derived from the subducted slab and effectively buffer fO(2) during fluxed-melting. The observed fO(2) variability in sub-arc mantle peridotites worldwide likely reflects the combination of fluxed- and adiabatic decompression melting in the mantle wedge.