^1,2S. R. Sutton,³A. J. Brearley,^3,4E. DobricĂ,¹A. Lanzirotti,¹M. Newville,⁵O. Tschauner
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13603]
¹Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, 60637 USA
²Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, 60637 USA
³Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131 USA
⁴Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, University of Hawai’i at Mānoa, Honolulu, Hawaii, 96822 USA
⁵Department of Geoscience, University of Nevada, Las Vegas, Nevada, 89154 USA
Published by arrangement with John Wiley & Sons

X‐ray absorption fine structure (XAFS) spectroscopy methods have been applied to focused ion beam (FIB) produced sections of olivine and pyroxene for determining the valence states of Ti, V, and Cr and inferring oxygen fugacities of formation for each element. High‐quality XAFS spectra were obtained for all three elements for analytical voxels of ~10 pg and usable spectra down to the pg level are achievable. The extraterrestrial samples studied here were olivine and pyroxene from chondrules in Semarkona (LL3.00), olivine from chondrules in Kainsaz (CO3.2), and an olivine and a pyroxene grain from two Antarctic micrometeorites (AMM). The general agreement between calculated thin section and FIB section valences strongly suggests that there is negligible alteration of Ti, V, and Cr valences during FIB sectioning. The inferred oxygen fugacities for the AMM olivine support an equilibrium igneous history similar to results seen for some achondrites. For the pyroxene, highly reduced Cr, coupled with relatively oxidized Ti, suggests an origin in a mildly metamorphosed chondritic parent body. These results demonstrate that this FIB and micro‐XAFS approach is promising for establishing the oxidation states of minute monomineralic grains of diverse extraterrestrial origins, including materials from sample‐return spacecraft, such as the Stardust, OSIRIS‐REx, Hayabusa, and Hayabusa2 missions.

¹Nicola J.Potts,^1,2Geoffrey D.Bromiley,³Richard A.Brooker
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.003]
¹School of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh, UK
²Centre for Science at Extreme Conditions, University of Edinburgh, UK
³School of Earth Sciences, University of Bristol, Bristol, UK
Copyright Elsevier

It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the ∼4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO2 to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH- in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification.