¹Benjamin H. Gruber,^1,2Robert W. Nicklas,¹James M. D. Day,¹Emily J. Chin,³Minghua Ren,⁴Rachel E. Bernard
Meteortics & Planetary Science (in Press) Open Access Link to Artivcle [https://doi.org/10.1111/maps.14179]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
²Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, Massachusetts, USA
³Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, Nevada, USA
⁴Department of Geology, Amherst College, Amherst, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Brachinites and brachinite-like achondrites are olivine-rich meteorites that represent materials after partial metal–silicate differentiation on multiple early Solar System bodies. Both meteorite types show macroscopic textures of olivine crystals, which make up >70 modal percent of their mineralogy. We investigated the orientations of olivine using electron backscatter diffraction (EBSD) and elemental compositions from paired brachinite-like achondrites and one brachinite. The olivine orientations are characterized by a strong concentration of [010] axes with maxima perpendicular to the foliation/layering and a concentration of [001] axes distributed in a girdle or, in a few samples, as point maxima. Trace element abundances of the olivine in these meteorites determined using laser ablation inductively coupled plasma–mass spectrometry have uniformly low concentrations of sodium (<300 μg g−1), aluminum (<70 μg g−1), and titanium (<40 μg g−1) that are distinct from olivine in chondrites or within terrestrial lavas. Instead, brachinite and brachinite-like olivine compositions broadly overlap those of olivine from melt-depleted mantle lithologies on Earth. Evidence from olivine trace element geochemistry, in conjunction with mineral fabrics, supports that these meteorites formed as melt residues on their host planetary body(ies).

^1,2Haiyang Xian,^1,2Jianxi Zhu,¹Yiping Yang,^1,2Shan Li,^1,2Jiaxin Xi,¹Xiaoju Lin,^1,2Jieqi Xing,¹Xiao Wu,^1,2Hongmei Yang,^1,2Hongping He,^2,3Yi-Gang Xu
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14174]
¹CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
²University of Chinese Academy of Sciences, Beijing, China
³State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
Published by arrangement with John Wiley & Sons

Charge disproportionation of ferrous iron has been considered as one of the mechanisms for the formation of metallic iron on the lunar surface. However, the detailed mechanism of the disproportionation reaction on the Moon is yet to be elucidated. We provide direct evidence for the ferrous disproportionation reaction that produces nano phase metallic iron (npFe0) during a rapid cooling process after splash melting from a lunar sample returned by China’s Chang’e-5 mission. Space weathering processes have resulted in the formation of three distinct zones at the rim of a pyroxene fragment, as observed through transmission electron microscopy. These zones, made up of splashed melts, newly formed melts from the substrate, and the mineral, are distinguished as I, II, and III. Quantitative analyses of the iron valence state by electron energy loss spectroscopy show that disproportionation reactions occurred in zone II at a low temperature of <570°C during a rapid cooling process. The reaction led to the production of α-structure npFe0 and Fe3+ reserve in the glass phase. The npFe0 produced by the disproportionation reaction has a larger grain size than those formed from solar wind irradiation, implying that micrometeoroid impacts mainly contribute to the darkening of visible and near-infrared reflectance. These findings reveal a novel rim structure by repeated space weathering and a universal formation mechanism of npFe0 during micrometeoroid impacts, suggesting that the disproportionation reaction could be widespread on airless bodies with impact-induced splash processes.