Presentation Information
[PPS12-P12]Quantitative analysis of iron valence states in phyllosilicate minerals using a Soft X-ray Emission Spectrometer
Shunki Kai1, *Wataru Fujiya1, Rei Kanemaru2, Takaaki Noguchi3, Naoya Imae4 (1.Graduate School of Science and Engineering, Ibaraki University, 2.Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3.Department of Geology and Mineralogy, Kyoto University, 4.National Institute of Polar Research)
Keywords:
Soft X-ray emission spectrometry,Iron valence state,Phyllosilicate minerals,Aqueous alteration,Carbonaceous chondrites
Phyllosilicate minerals in carbonaceous chondrites are among the most important phases recording aqueous alteration processes and redox environments in the early Solar System. In particular, the iron valence state (Fe2+/ΣFe) constrains oxygen fugacity (fO2), fluid pH, and the mode of water transport within their parent bodies. However, existing XANES and Mössbauer methods for quantifying iron valence ratios are limited by sample preparation and facility dependence, motivating the development of a practical technique for micrometer-scale analysis.
In this study, we employed soft X-ray emission spectroscopy (SXES), which allows relatively simple sample preparation and in situ analysis. SXES can be installed on an electron probe microanalyzer (EPMA) and provides high-energy-resolution Fe-L spectra sensitive to iron valence. We tested two quantitative approaches based on EPMA-SXES Fe-L spectra: (1) estimation from the peak position of self-absorption spectra (SAS method), and (2) estimation from the integrated Fe-Lβ/Fe-Lα intensity ratio (Lβ/Lα method). Calibration curves were constructed using six standards with known iron valence ratios (forsterite, fayalite, babingtonite, Two cronstedtites, and laihunite), and the effects of analytical conditions and mineral structure on reproducibility and accuracy were evaluated.
Although the SAS method is theoretically sensitive to iron valence, uncertainty of the Fe-Ll spectrum used for energy calibration increases SAS peak-position uncertainty, particularly in samples containing <10 wt% Fe. In contrast, the Lβ/Lα method showed high reproducibility independent of mineral species, yielding a stable linear relationship between Lβ/Lα intensity ratios and iron valence ratios across multiple sessions (R2 > 0.95).
We applied this approach to matrix in the Ivuna (CI), Aguas Zarcas (CM), and Tarda (CC-ung) carbonaceous chondrites. The Lβ/Lα method consistently gave Fe valence ratios < 0.4, indicating Fe3+-rich matrices. Tarda exhibited spatial variability, likely reflecting heterogeneous aqueous alteration. The SAS method reproduced overall trends but with larger uncertainties, making it difficult to distinguish between meteorites.
These results demonstrate that Fe valence estimation using EPMA-SXES Fe-L spectra is a robust tool for assessing the redox state of phyllosilicate minerals in carbonaceous chondrites. Practically, we suggest the Lβ/Lα method as the primary indicator, with the SAS method as a complementary check.
Future work should expand calibration standards over a wider range of iron valence ratios and analyze meteorites with more diverse alteration degrees to investigate the relationship between Fe valence states and alteration degrees.
In this study, we employed soft X-ray emission spectroscopy (SXES), which allows relatively simple sample preparation and in situ analysis. SXES can be installed on an electron probe microanalyzer (EPMA) and provides high-energy-resolution Fe-L spectra sensitive to iron valence. We tested two quantitative approaches based on EPMA-SXES Fe-L spectra: (1) estimation from the peak position of self-absorption spectra (SAS method), and (2) estimation from the integrated Fe-Lβ/Fe-Lα intensity ratio (Lβ/Lα method). Calibration curves were constructed using six standards with known iron valence ratios (forsterite, fayalite, babingtonite, Two cronstedtites, and laihunite), and the effects of analytical conditions and mineral structure on reproducibility and accuracy were evaluated.
Although the SAS method is theoretically sensitive to iron valence, uncertainty of the Fe-Ll spectrum used for energy calibration increases SAS peak-position uncertainty, particularly in samples containing <10 wt% Fe. In contrast, the Lβ/Lα method showed high reproducibility independent of mineral species, yielding a stable linear relationship between Lβ/Lα intensity ratios and iron valence ratios across multiple sessions (R2 > 0.95).
We applied this approach to matrix in the Ivuna (CI), Aguas Zarcas (CM), and Tarda (CC-ung) carbonaceous chondrites. The Lβ/Lα method consistently gave Fe valence ratios < 0.4, indicating Fe3+-rich matrices. Tarda exhibited spatial variability, likely reflecting heterogeneous aqueous alteration. The SAS method reproduced overall trends but with larger uncertainties, making it difficult to distinguish between meteorites.
These results demonstrate that Fe valence estimation using EPMA-SXES Fe-L spectra is a robust tool for assessing the redox state of phyllosilicate minerals in carbonaceous chondrites. Practically, we suggest the Lβ/Lα method as the primary indicator, with the SAS method as a complementary check.
Future work should expand calibration standards over a wider range of iron valence ratios and analyze meteorites with more diverse alteration degrees to investigate the relationship between Fe valence states and alteration degrees.