Abstract
We report application of the flank method using the electron microprobe to a suite of twelve (Mg,Fe)O samples with composition 2-47 wt% Fe and Fe3+/ΣFe=1 to 11%, where Fe3+/ΣFe was determined independently using Mössbauer spectroscopy on the same grains used for the flank method measurements. A calibration curve of the form Fe2+=A+B× (ΣFe)2+C× (Lβ/Lα) was fit to the data and gave excellent agreement between Fe3+/ΣFe calculated from the flank method and Fe3+/ΣFe determined using Mössbauer spectroscopy. We found the method to be sufficiently sensitive to determine meaningful variations in Fe3+/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 Mössbauer spectroscopy. Flank method measurements on the same suite of single crystals showed enhanced Fe3+/ΣFe values, consistent with the presence of magnesioferrite even though the grains were too small to be resolved by conventional electron microprobe measurements.
Original language | English (US) |
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Pages (from-to) | 1249-1257 |
Number of pages | 9 |
Journal | Contributions to Mineralogy and Petrology |
Volume | 162 |
Issue number | 6 |
DOIs | |
State | Published - Dec 2011 |
Funding
Acknowledgments We are grateful to Stephen Mackwell for supplying some of the samples examined in this study, to Hugh O’Neill for valuable advice concerning (Mg,Fe)O synthesis, to Heidi Höfer for her support on the flank method calibration, to Detlef Krauße for his technical support at the electron microprobe, to Nobuyoshi Mi-yajima for TEM investigations and to Florian Heidelbach for SEM analyses. The manuscript was significantly improved by the comments of Andrew Berry and an anonymous reviewer. ML received financial support from the European Commission under the Marie Curie Action for Early Stage Training of Researchers within the 6th Framework Programme (contract number MEST-CT-2005-019700). SDJ is supported in part by the US NSF (EAR-0748707) and by the David and Lucile Packard Foundation.
Keywords
- Electron microprobe
- Ferropericlase
- Flank method
- Iron oxidation state
ASJC Scopus subject areas
- Geophysics
- Geochemistry and Petrology