Fe2+ and Fe3+ quantification by different approaches and f(O2) estimation for Albanian Cr-spinels

Fourteen Cr-spinels from Albanian ophiolites were examined. Fe2+/Fe2+ ratios were obtained by Fe-57 Mossbauer 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. Fell contents by EMPA, calculated from spinel stoichiometry, are almost always underestimated with respect to those obtained by MS. Moreover, Fell 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 7 on sample provenance, but suggest a strong dependence on spinel composition. Chemical data of Albanian spinel and associated olivine were used to estimate F-O2 via oxygen geobarometry. Notably, f(O2) values calculated on basis of the EMPA data are always largely underestimated with respect to those obtained on basis of MS data. However, f(O2) values calculated on basis of MS-RT data are equal to or higher than (up to 0.2 log units) f(O2) values based on MS-LT data. The increase in f(O2) 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.