Upper mantle oxidation state: Ferric iron contents of Iherzolite spinels by 57Fe Mössbauer spectroscopy and resultant oxygen fugacities

The ferric iron contents of spinels from 30 spinel Iherzolite xenoliths have been measured by ⁵⁷Fe Mössbauer spectroscopy. The samples are widely dispersed in geographic and tectonic setting, coming from Southwest United States (San Carlos, Kilbourne Hole), Japan (Ichinomegata), Massif Central, France (Mont Briançon) and Central Asia (Tariat Depression, Vitim Plateau). The total range of Fe₃O₄ contents of the spinels is from 1.7 to 5.2 mol% with an uncertainty of 0.2 to 0.3 mol%. These data were used to calculate the oxygen fugacities recorded by the spinel Iherzolites using the oxygen thermobarometer 6 Fe₂SiO₄ + O₂ = 3 Fe₂Si₂O₆ + 2 Fe₃O₄. olivine orthopyroxene spinel The Fe(III) contents of the spinels translate to oxygen fugacities which, at 15 kb, range between 1.7 log units below and 1.2 log units above FMQ using either the Mattioli and Wood (1988) or O'neill and Wall (1987) version of Fe₃O₄ activity. There are distinct regional differences f{hook}O₂, the specimens from SW U.S.A. and Central Asia exhibiting values from slightly above FMQ to 1.5 log units below FMQ. At an estimated pressure of 15 kb, these values overlap with the f{hook}O₂-T field of MORB glasses, indicating, in agreement with trace element abundances, that many of these samples are related to the MORB source region. Samples from Ichinomegata and Mont Briançon are all above the MORB range, however, suggesting progressive oxidation related to subduction processes. All of our samples give oxygen fugacities more than 2 log units above IW, implying that CHO fluids in the upper mantle are dominated by CO₂ and H₂O and that CH₄ is a minor (< 10%) component. A detailed comparison of Fe(III) contents determined by Mössbauer spectroscopy and those obtained from microprobe analysis indicates that the latter are sufficiently precise (± 0.002 fX_Fe3O₄) but, in general, too inaccurate for oxygen thermobarometry. Use of Mössbauer-analyzed spinels as microprobe standards enables accuracy to approach precision, however, and appropriate standards are available on request.