Determination of ferrous iron in biotite by EPMA using the flank method: development of a mineral-specific calibration

The oxidation state of iron in biotite provides key information on redox conditions, magma evolution, and fluid–rock interactions in igneous, metamorphic, and hydrothermal systems, yet routine determination of Fe2+ and Fe3+ by electron probe microanalysis (EPMA) remains difficult. This study presents a biotite-specific calibration of the flank method for estimating Fe2+ from Fe Lβ/Lα intensity ratios. Rather than defining flank positions from garnet end-members, as in the conventional approach, flank positions were established directly from biotite spectra, exploiting the comparatively narrow Fe3+/ΣFe range of biotite to achieve more consistent spectral behaviour. Twelve biotite samples spanning a wide range of total iron content (8.29–22.61 wt% ΣFeO) were analysed by EPMA and independently characterised by wet chemistry to calibrate and test the method. A simple linear regression of Fe2+ against Lβ/Lα produced a strong correlation with wet-chemistry values (R2 = 0.95, average error 0.47 wt%), and incorporating total iron (ΣFe) into a multiple regression model improved the prediction of Fe2+/ΣFe, accounting for matrix-dependent effects associated with total Fe concentration. Leave-one-out cross-validation was applied to evaluate the performance of both models, and independent validation against a well-characterised reference biotite (LP-6) yielded Fe2+ estimates within 0.4–0.6 wt% of the reported composition. These results demonstrate that mineral-specific calibration of the flank method, combined with controlled instrumental conditions, offers a practical, and in-situ alternative to Mossbauer spectroscopy or synchrotron XANES for constraining Fe2+/Fe3+ in biotite, with potential application to other Fe-bearing silicates and to broader petrogenetic, ore-deposit, and metamorphic studies in which iron oxidation state provides information on mineral-forming conditions.

Chemical Geology

Chemical Geology

726

1872-6836

N/A

N/A

18