Precise Mössbauer 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, Fe31 in coexisting minerals is either assumed to be zero or is calculated from electron microprobe analyses (EMPA) based upon stoichiometry and charge balance. Geothermobarometers that involve Fe21-Mg21 exchange can be significantly affected by either neglecting Fe31 or using incorrect values. Ratios of Fe31/SFe in garnet and clinopyroxene measured by a Mössbauer milliprobe were compared to those calculated from EMPA of garnet and clinopyroxene from eclogite xenoliths from the Udachnaya kimberlite in Yakutia. The effects of Fe31 contents in garnet and clinopyroxene on temperature estimations were evaluated. The following Fe31/SFe (in at%) values were obtained (EMPA/Mössbauer): Gt 5 9.4/ 6.0; 11.5/7.0; 19.4/16.0; and 24.7/15.0; Cpx 5 22.0/22.9; 34.2/22.0. The effects of Fe31 in clinopyroxenes on calculated temperatures are illustrated by taking eclogitic clinopyroxene compositions and changing contents 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 Fe31/SFe from 31.6 to 47.1%, thereby decreasing the calculated temperature from 1026 to 941 8C. Various Fe31/SFe values for garnet and clinopyroxene were also tested for their effects on calculated temperatures: for clinopyroxene, T decreases with increasing Fe31/SFe whereas for garnet, T increases with increasing Fe31/SFe. This compensation effect between garnet and clinopyroxene moderates the variation in temperature estimations of eclogites based on Fe31 corrected vs. uncorrected microprobe analyses. Little correlation exists between EMPA-calculated and Mössbauer-measured Fe31/SFe values for these mantle-derived garnets and clinopyroxenes. Even a small relative error in Fe31 may significantly change calculated temperatures of equilibration, seriously affecting petrologic interpretations. In particular, uncertainty in Fe31 calculated from EMPA of silicate minerals leads to serious questions with regard to KD values obtained from natural assemblages.