Determination of the oxidation state of iron in calcic pyroxene using the electron microprobe flank method

Pyroxene is an important carrier of ferric iron in basalt and the upper mantle. Understanding the influence of pyroxene crystallization on the oxygen fugacity of magma relies on accurate knowledge of the oxidation state of iron, expressed as the Fe³⁺/ΣFe ratio, in pyroxene. To accurately determine the Fe³⁺/ΣFe ratio in pyroxene using an electron probe microanalysis, we present nine natural pyroxene samples, including one aegirine, one hedenbergite, one diopside, and six augites, for the calibration of the flank method for pyroxene. The aegirine sample is rich in the aegirine end-member with an Fe³⁺/ΣFe ratio of 0.98 ± .01 (1σ), while the hedenbergite sample is rich in the hedenbergite end-member and free of ferric iron. The augite and diopside samples contain variable ferric iron, with the Fe³⁺/ΣFe ratios varying from 0.21 to 0.39. Based on the flank positions of FeLα and FeLβ determined by natural andradite and almandine, we measured the Fe Lβ/Lα ratios at the flank positions for the pyroxene samples. The results demonstrate a positive linear relationship between the Fe Lβ/Lα ratios and the Fe²⁺ content of the pyroxene samples. The Fe²⁺ contents and Fe³⁺/ΣFe ratios of the pyroxene samples, obtained through a multiple linear regression equation, align closely with those determined by Mössbauer spectroscopy. This method yields Fe²⁺ content and Fe³⁺/ΣFe ratios with an error of ±0.3 wt.% and ±0.06, respectively, for calcic pyroxene containing 7 wt.% total FeO. These well-characterized natural pyroxene samples can serve as reference materials for determining the Fe³⁺/ΣFe ratio in unknown calcic pyroxene.