The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis

We have established a method to determine the oxidation state of Fe using the electron microprobe-the flank method. This method has the possibility to analyze simultaneously the major-element chemistry on the same spot. The method is based on FeL X-ray emission spectra that show a concomitant change of both the intensity and the wavelength of the soft FeL alpha and FeL beta emission lines. The method has been developed for garnets using well-characterized synthetic samples, and an empirical correction scheme has been established for self-absorption of the FeL X-ray emission. This enables the unambiguous correlation of measured intensity ratios of FeL alpha/FeL beta X-ray emission to the oxidation state in garnet. The flank method can be used with routine microprobe settings (e.g., 15 kV, 60 nA) and, for garnets, with minimum beam size. This way, Fe3+/Sigma Fe and major and trace elements can be determined within 10 minutes on the microscale. The resulting accuracy of Fe3+/Sigma Fe is then +/- 0.04 for garnets at 10 wt% total Fe. Application to natural garnet megacrysts with 8 wt% total Fe and Fe3+ known from Mossbauer spectroscopy yields excellent agreement between the two methods. For practical application, the calibration can be achieved by using three natural standards and fitting the data to Fe2+ and Fe3+ with linear equations. Through repeated measurements on homogeneous samples, the error in Fe3+/Sigma Fe becomes approximately +/- 0.02 (1 sigma). Optimization of analytical parameters such as beam current, measurement time, and repetition of analyses, will further lower the uncertainty.