Abstract
The aegirine end-member (NaFe3+Si2O6) in clinopyroxenes resulted from incorporation of Fe3+ into the mineral structure effects the accuracy of reconstruction of the P-T conditions in the high-grade metamorphic rocks and also allows evaluation the redox conditions of their formation. As a rule, the content of this end-member in clinopyroxenes is evaluated based on the crystal chemical recalculations of microprobe analyses. However, in some publications on eclogites, the results of recalculations of clinopyroxenes were compared with the data of Mössbauer spectroscopy. Significant difference was revealed between the measured and calculated Fe3+/ΣFe ratios, that can significantly affect the results of geothermometry. This paper presents the results of the Mössbauer spectroscopy measurements of clinopyroxene fractions separated from three samples of garnet-clinopyroxene granulites from the Udachnaya kimberlite pipe. The ratios Fe3+/ΣFe = 0.22–0.26 measured in clinopyroxenes correspond to 6–10 mol. % aegirine. These estimates are in good agreement with the values obtained for clinopyroxenes from the same samples by the recalculation of microprobe analyzes using the charge balance method. Following to this conclusion, we believe that crystal chemical recalculations of microprobe analyzes of clinopyroxenes from non-eclogitic rocks make it possible to correctly estimate the Fe3+ content in them. Similar recalculation of microprobe analyzes of clinopyroxenes from crustal xenoliths from other localities, as well as from ferrobasalts of the continental flood basalts provinces, ferrodolerite dikes, and gabbroid xenoliths (similar in bulk chemical composition to many lower-middle-crustal xenoliths) revealed significant amounts of previously unaccounted aegirine in them (up to 13 and 4–9 mol. %, respectively), that unleashes the potential for the reconstruction of redox conditions in many rocks.
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