A spinel-melt oxybarometer is established by combining the equation of Ariskin and Nikoraev (1996), describing the Fe
2+-Fe
3+ partitioning between spinel and mafic melt, and that of Sack et al. (1981), relating the Fe
3+/Fe
2+ ratio and oxygen fugacity (
fO2) in silicate melt. Published experimental datasets for spinel-melt equilibria under controlled
fO2 and temperature conditions were compiled, used for evaluating the reliability of the oxybarometer; the oxybarometer represents experimental
fO2 conditions within ± 1 log unit (1σ). The oxybarometer was applied to spinel inclusions crystallized with olivine phenocrysts at liquidus temperatures in Cenozoic alkali basaltic magmas from the Higashi-Matsuura district, NW Kyushu, Japan. The results indicate that the redox states of the alkali basaltic magmas at liquidus temperatures were ca. 0.5 log unit below the quartz-magnetite-fayalite (QMF) buffer. The redox states are consistent with previously published results estimated from lithospheric mantle-derived spinel peridotite xenoliths from NW Kyushu, and are within the range of abyssal peridotites, suggesting that
fO2 in upper mantle beneath the NW Kyushu back arc region was similar to that of MORB mantle.
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