¹Yang Bai et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [DOI: 10.1016/j.gca.2026.09.038]
¹State Key Laboratory of Continental Evolution and Early Life, NWU-HKU Joint Center of Earth and Planetary Sciences, Department of Geology, Northwest University, Xi’an 710069, China

Copyright Elsevier

The composition of the lunar deep interior remains debated, as remote sensing observations indicate that impact-excavated materials from within and around large basins contain both olivine and low-Ca pyroxene (LCP). Here we show a partially melted exotic troctolitic clast from Chang’e-5 lunar soils that records impact-induced mineral transformations. The exotic clast consists of forsteritic olivine grains with magnesium-rich rims and pyroxene crystals embedded in a feldspathic glassy matrix. Pyroxene crystals occur as elongated aggregates, with dendritic crystallites growing along the grain edges. The elongated pyroxene crystals show distinct zoning, with Fe-enriched, low-Ca cores and Al-enriched, high-Ca rims, while the dendritic crystallites are compositionally identical to the rims. Elemental mass-balance considerations suggest that impact-induced melting and recrystallization of olivine and anorthite generated the observed LCP–Al-rich high-Ca pyroxene (HCP) assemblage, with Si released during anorthite transformation compensating for the substantial Si consumption during LCP formation. These observations indicate that the LCP formed through impact-induced partial melting of an olivine-rich troctolite under rapid cooling conditions, rather than being a primary phase excavated from the lunar deep interior. Our findings demonstrate that LCP signatures observed around large impact basins, e.g., Imbrium and South Pole-Aitken basins, may originate from impact-modified olivine-rich lithologies, rather than being directly excavated from the deep interior. These results raise the possibility that such transformations may be a widespread process associated with large-scale impacts on the Moon and have important implications for the interpretation of remote sensing data and constraints on the composition of the lunar interior.

^1,2Alan E. Rubin
Meteoritics & Planetary Science (in Press)
Open Access Link to Article [DOI: 10.1111/maps.70244]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA

Published by arrangement with John Wiley & Sons

Major chondrite groups (H, L, LL, R, EH, EL, CK) with some members of petrologic type 5 and 6 also have some members of types 3 and 4; they do not have members of type 1 or 2. Their parent bodies had onion-shell structures and were internally heated by the decay of ²⁶Al. These bodies experienced significant thermal metamorphism but only minor aqueous alteration. Major chondrite groups (CI, CM, CV, CR) that have at least some members of petrologic type 1 or 2 have no members of petrologic type 5 or 6. Their parent bodies were subjected to extensive aqueous alteration but little thermal metamorphism. As shown previously, there is a positive correlation between petrologic type and shock stage in ordinary chondrites. Their parent bodies were disrupted and reassembled while still hot; subsequent stochastic collisions caused the exposed hotter materials (i.e., outcrops of higher petrologic types) to develop more-intense shock features. Although CK chondrites encompass the entire range of metamorphic petrologic types (3–6), they exhibit no correlation between petrologic type and shock stage. This may indicate the CK parent body was relatively small, cooled quickly, and was disrupted and reassembled into a rubble pile after it had already cooled down. Stochastic collisions into surface regions that had already cooled down would have resulted in every CK petrologic type developing about the same mean shock stage.