M. Darby Dyar: Application of Multivariate Analysis Techniques to Quantitative Interpretation of Valence States in XAS

The study of extraterrestrial samples returned from the Moon, asteroids, and via meteorites has the potential to inform problems relating to the oxidation states of multivalent cations and the distribution of oxygen in our Solar System. For more than 25 years, our group has worked to develop XAS methods for analyzing the partial pressure of oxygen in formation environments of tiny returned samples. This problem has application to all materials in which quantitative measurement of valence states is needed. This talk will provide a brief history of measurements of oxidation state in minerals, and discuss the particular challenges of undertaking calibration data needed for anisotropic materials using a polarized synchrotron beam. We will show how multivariate analysis techniques such as partial least squares can be used to predict valence state and oxygen partial pressure at one-micron scales, and enable mapping of small grains by selective choice of specific channels. These new methods enable understanding of processes as diverse as lunar volcanic eruptions and lightning strikes in terrestrial deserts.

References:

1. Dyar et al. (2016) Accurate predictions of iron redox state in silicate glasses: A multivariate approach using x-ray absorption spectroscopy. Amer. Mineral., 101, 744-748. https://doi.org/10.2138/am-2016-5555
2. Dyar et al. (2016) Use of multivariate analysis for synchrotron micro-XANES analysis of iron valence state in amphiboles. Amer. Mineral., 101, 1171-1189. https://doi.org/10.2138/am-2016-5556
3. McCanta et al. (2017) In situ measurement of ferric iron in lunar glass beads using Fe-XAS. Icarus, 285, 95-102, 10.1016/j.icarus.2016.12.029. https://doi.org/10.2138/am-2016-5556
4. Lanzirotti et al. (2018) Accurate predictions of microscale oxygen barometry in basaltic glasses using vanadium K-edge x-ray absorption spectroscopy: A multivariate approach. Amer. Mineral., 103, 1282-1297.https://doi.org/10.2138/am-2018-6319
5. Roberts et al. (2019) Oxidation state of iron in fulgurites and trinitite: Implications for redox changes during abrupt high-temperature and pressure events. Geochim. Cosmochim. Acta, 266, 332-350. https://doi.org/10.1016/j.gca.2019.08.021