The mantle redox state; An unfinished story?

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Abstract
We review mantle redox models and present new data for xenolithic and megacrystic intrinsic oxygen fugacity (IOF) studies from varied geologic settings. The roles of fluid inclusions, carbon, autoreduction, auto-oxidation, Ti3+, crystal defects, disproportionation, metasomatism, and gravity are examined with regard to their possible influences on redox data. In several IOF studies, the geothermometric determination for a multimineral sample yields very close temperature concordance with totally different geothermometric techniques; these specific studies mandate some confidence in the IOF fO2 values obtained. Also of high confidence are the IOF data that come from nearly flawless, gem-quality megacrysts (GQ) of various silicates. Both types of these high-confidence IOF data indicate that the redox state of the mantle is nearer the wüstite-iron (WI) buffer than the quartz-fayalite-magnetite (QFM) buffer. The fO2 data of the ilmenite-containing xenolithic assemblages that have either been calculated (Eggler, 1983), IOF-measured (Arculus and others, 1984), or gas mixture-equilibrated (McMahon, 1984) have all shown reasonable overlap in fO2-T values, and all indicate that these samples come from a mantle region more like (QFM). The ilmenitic xenoliths subjected to IOF analysis in our laboratory have exhibited auto-oxidation and therefore are difficult to interpret unequivocally. The high-confidence IOF data do argue strongly for mantle redox inhomogeneity, but much more work is needed to establish whether there is a general systematic redox decrease with depth into the mantle, on which the observed redox inhomogeneity is superimposed.

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