1 American Mineralogist Manuscript #4837 – Revision 1 2 3 4 The petrogenesis of impact basin melt rocks in lunar meteorite Shişr 161 5 6 Axel Wittmann1,*, Randy L. Korotev1, Bradley L. Jolliff1, Thomas J. Lapen2, and 7 Anthony J. Irving3 8 9 1 Department of Earth and Planetary Sciences, Washington University St. Louis, Campus 10 Box 1169, 1 Brookings Dr., St. Louis, MO 63130-4899, USA 11 2 Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun 12 Road, Houston, Texas 77004. 13 3 Department of Earth and Space Sciences, University of Washington, 4000 15th Avenue 14 NE, Seattle, Washington 98195. 15 *corresponding author e-mail address:
[email protected] 16 Page 1 of 53 17 ABSTRACT 18 This study explores the petrogenesis of Shişr 161, an immature lunar regolith 19 breccia meteorite with low abundances of incompatible elements, a feldspathic affinity, 20 and a significant magnesian component. Our approach was to identify all clasts >0.5 mm 21 in size in a thin section, characterize their mineral and melt components, and reconstruct 22 their bulk major and minor element compositions. Trace element concentrations in 23 representative clasts of different textural and compositional types indicate that the clast 24 inventory of Shişr 161 is dominated by impact melts that include slowly cooled cumulate 25 melt rocks with mafic magnesian mineral assemblages. Minor exotic components are 26 incompatible-element-rich melt spherules and glass fragments, and a gas-associated 27 spheroidal precipitate. Our hypothesis for the petrologic setting of Shişr 161 is that the 28 crystallized melt clasts originate from the upper ~1 km of the melt sheet of a 300 to 500 29 km diameter lunar impact basin in the Moon’s feldspathic highlands.