Upper mantle oxidation state

Ferric iron contents of Iherzolite spinels by 57Fe Mössbauer spectroscopy and resultant oxygen fugacities

Bernard J. Wood*, David Virgo

*Corresponding author for this work

    Research output: Contribution to journalArticle

    250 Citations (Scopus)

    Abstract

    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 fXFe3O4) 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.

    Original languageEnglish
    Pages (from-to)1277-1291
    Number of pages15
    JournalGeochimica et Cosmochimica Acta
    Volume53
    Issue number6
    DOIs
    Publication statusPublished - 1989

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