Microanalysis of the iron oxidation state in (Mg,Fe)0 and application to the study of microscale processes

摘要

We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)0 samples with composition 2-47 wt% Fe and Fe～(3+)/ΣFe = 1 to 11%, where Fe～(3+)/ΣFe was determined independently using Mossbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe~(2+) = A + B x (ΣFe)~2 + C x (Lβ/Lα) was fit to the data and gave excellent agreement between Fe～(3+)/ΣFe calculated from the flank method and Fe～(3+)/ΣFe determined using Mossbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe～(3+)/ΣFe for geophysically relevant compositions of (Mg,Fe)O (<25 wt% Fe), and calibration parameters remained constant within experimental uncertainty over the course of the entire study (20 months). Flank method measurements on an inhomogeneous sample of synthetic (Mg,Fe)O showed evidence of diffusion processes resulting from rupture of the capsule during the high-pressure experiment and the possibility to measure Lβ/Lα variations with a spatial resolution of a few microns. We detected the presence of exsolved magnesioferrite in a suite of (Mg,Fe)O single crystals using transmission electron microscopy and Mossbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe～(3+)/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.