Precise Mossbauer milliprobe determination of ferric iron in rock-forming minerals and limitations of electron microprobe analysis

For estimations of P-T conditions of igneous and metamorphic rocks, Fe3+ in coexisting minerals is either assumed to be zero or is calculated from ele ctron microprobe analyses (EMPA) based upon stoichiometry and charge balanc e. Geothermobarometers that involve Fe2+-Mg2+ exchange can be significantly affected by either neglecting Fe3+ or using incorrect values. Ratios of Fe 3+/Sigma Fe in garnet and clinopyroxene measured by a Mossbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene fro m eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe3+ contents in garnet and clinopyroxene on temperature estimations wer e evaluated. The following Fe3+/Sigma Fe (in at%) values were obtained (EMPA/Mossbauer): Gt = 9.4/6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx = 22.0/22.9; 34.2/22 .0, The effects of Fe3+ in clinopyroxenes on calculated temperatures are il lustrated by taking eclogitic clinopyroxene compositions and changing conte nts of certain elements within the range of standard deviations for EMPA of those particular elements. Increasing Na2O contents from 5.67 to 5.74 wt% (<2.0% relative error) would lead to increasing Fe3+/Sigma Fe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 degre es C. Various Fe3+/Sigma Fe values for garnet and clinopyroxene were also t ested for their effects on calculated temperatures: for clinopyroxene, T de creases with increasing Fe3+/Sigma Fe whereas for garnet, T increases with increasing Fe3+/Sigma Fe. This compensation effect between garnet and clino pyroxene moderates the variation in temperature estimations of eclogites ba sed on Fe3+ corrected vs, uncorrected microprobe: analyses. Little correlation exists between EMPA-calculated and Mossbauer-measured Fe 3+/Sigma Fe values for these mantle-derived garnets and clinopyroxenes. Eve n a small relative error in Fe3+ may significantly change calculated temper atures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe3+ calculated from EMPA of silicate minerals leads to serious questions with regard to K-D values obtained from natural assemblages.