Computer Science
Scientific paper
Jun 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989gecoa..53.1277w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 53, Issue 6, pp.1277-1291
Computer Science
43
Scientific paper
The ferric iron contents of spinels from 30 spinel Iherzolite xenoliths have been measured by 57 Fe 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 Fe 3 O 4 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 Fe 2 SiO 4 + O 2 = 3 Fe 2 Si 2 O 6 + 2 Fe 3 O 4 . 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 Fe 3 O 4 activity. There are distinct regional differences f O 2 , 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 O 2 - 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 C---H---O fluids in the upper mantle are dominated by CO 2 and H 2 O and that CH 4 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 Fe 3 O 4 ) 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.
Virgo David
Wood Bernard J.
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