The conventional application of an electron microprobe is the in situ elemental quantitative analysis of solid material on the microscale, down to trace element concentrations. We present a “non-conventional” application for the in situ determination of the oxidation state by the electron microprobe.
The Fe oxidation state of minerals and glasses in rocks reflects the prevailing oxygen fugacity during their formation. It is the determining factor for volatile speciation during metamorphism and magmatic differentiation. Specifically, the in situ speciation of Fe on the microscale is relevant for many applications in Earth Sciences, since natural minerals are generally small and often zoned and high-pressure experimental run products are commonly very small with 10-15 µm.
Various methods are available for the determination of the Fe oxidation state but most lack a high spatial resolution. We developed the „flank method“ for the in situ determination of the iron oxidation state [1]. It is based on the accurate intensity measurement at two positions on the flanks of the FeLa and FeLb emission lines and makes thus use of the systematic change of the intensity and wavelength of the FeL lines with the iron oxidation state. The method offers the huge advantage that the major element composition can be measured simultaneously.
The flank method has been calibrated for the garnet structure with a large number of different synthetic and natural garnets [1]. Various applications will be presented with a specially challenging application, the determination of the Fe oxidation state at low Fe contents in peridotitic mantle garnets. We optimized the method so that only 4 natural garnet standards with low total Fe contents are necessary to calibrate the flank method. The errors achieved are as with Mössbauer spectroscopy in the order of ± 0.01 (1s). The calibration for clinopyroxenes is currently in progress.