Electron probe microanalysis of Fe2+/ΣFe ratios in calcic and sodic-calcic amphibole and biotite using the flank method

Abstract

We present an analytical approach to determine the Fe2+/ΣFe ratio in amphiboles and biotites using electron probe microanalysis (EPMA). The “flank method” applied in this study is based on the fact that FeL line spectra show different peak position and intensity for Fe2+ and Fe3+, which is associated with a resonant process involving both X-ray absorption and emission. The flank positions for Lα and Lβ, which correspond to sites with minimal Fe3+L3 absorption and maximal Fe2+L2 absorption respectively, are determined based on the L-line spectra difference of two Fe-rich ferric and ferrous garnet endmembers, i.e. andradite and almandine. Spectra intensities measured at the Lα and Lβ flank positions for selected amphibole and biotite references can be quantitatively correlated with their Fe2+ weight contents by a linear relation: Lβ/Lα = a + b*Fe2+, in which a and b are constants calibrated upon reference materials. It is emphasized that the constants a and b may vary significantly for different mineral groups and analytical conditions. Thus, a calibration is necessary for each analytical session. Our tests show that the potential beam damage during EPMA that may induce oxidation of the measured material can be minimized by using a large beam size (e.g., 20 μm). The Fe2+/ΣFe ratios determined using the EPMA flank method in collected natural calcic and sodic-calcic amphiboles (FeOT = 2.4–12.7 wt%, Fe2+/ΣFe = 0.45–75) and biotites (FeOT = 4.3-28.2 wt%, Fe2+/ΣFe = 0.67–0.87) are consistent within an error of ±0.1 with the data obtained from the colorimetric wet chemistry method. The EPMA flank method has the potential to be widely applied for accurate in-situ determination of Fe oxidation states in a wide range of minerals, provided that references are available.