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Fe2+ and Fe3+ quantification by different approaches and fO₂ estimation for Albanian Cr-spinels
-
Marco Quintiliani
and
Giorgio Graziani
Published/Copyright:
March 31, 2015
Abstract
Fourteen Cr-spinels from Albanian ophiolites were examined. Fe2+/Fe3+ ratios were obtained by 57Fe Mössbauer spectroscopy (MS) and compared with ratios retrieved by electron microprobe analyses (EMPA). MS spectra were collected at both 298 K (RT) and 77 K (LT), and fitted using various interpretative models. Fe3+ contents by EMPA, calculated from spinel stoichiometry, are almost always underestimated with respect to those obtained by MS. Moreover, Fe3+ contents by MS-RT are shown to be somewhat overestimated with respect to those by MS-LT, which are proved to be the most reliable. On basis of MS results, Albanian Cr-spinels proved to be non-stoichiometric, with an oxidation degree, z, ranging from 4 to 49%. Our results indicate no dependence of z on sample provenance, but suggest a strong dependence on spinel composition. Chemical data of Albanian spinel and associated olivine were used to estimate fO₂ via oxygen geobarometry. Notably, fO₂ values calculated on basis of the EMPA data are always largely underestimated with respect to those obtained on basis of MS data. However, fO₂ values calculated on basis of MS-RT data are equal to or higher than (up to 0.2 log units) fO₂ values based on MS-LT data. The increase in fO₂ responsible for spinel oxidation was tentatively estimated to be less than one log unit for poorly oxidized samples, but up to 6 log units for the most oxidized samples.
Keywords : Analysis; chemical (mineral); chromite; olivine; Mössbauer spectroscopy; thermobarometry; olivine-spinel
Received: 2005-6-7
Accepted: 2005-11-7
Published Online: 2015-3-31
Published in Print: 2006-5-1
© 2015 by Walter de Gruyter Berlin/Boston
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Keywords for this article
Analysis;
chemical (mineral);
chromite;
olivine;
Mössbauer spectroscopy;
thermobarometry;
olivine-spinel
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