Petrology And Zirconology Of The Regolith Breccia Of Lunar Meteorite Sayh Al Uhaymir 169

The magmatism and impact history of the Moon,especially the KREEP(enriched in potassium,rare earth elements,phosphorous,and other incompatible elements)associated magmatism and the pre-3.9 Ga impact events,are the key questions shedding lights on the early evolutions of the Earth,other planets,and differentiated asteroids.Lunar meteorites are important supplements to lunar samples returned by the Apollo,Luna,and Chang'e programs.Since the source of lunar meteorites is random,they may have new rock types or chemical-anomaly fragments bearing additional information on lunar magmatism and impact history.Thus,here we study the petrology,geochemistry,and zirconology of a lunar meteorite SaU 169.SaU 169,one of the most KREEP-rich lunar meteorites,is composed of impact melt breccia and polymict regolith breccia.In the regolith portion,there are diverse lithic and mineral fragments derived from(i)lunar mare basalts,mainly low-Ti,and some have aluminous,high potassium,and ferroan nature;(ii)nonmare igneous rocks dominated by magnesian and alkali suite types,with minor ferroan anorthosite;and(iii)impactites of non-mare materials and minor mare materials.Some impactites have high K₂O abundances and high-K KREEP-like REE patterns,indicating that KREEP-rich materials have been mixed into their precursors.Lunar zircons,crystallized solely from KREEP-rich melts(typically?3.9-4.4 Ga ago)and experienced intense impact events afterward,are optimal objects for studying the early KREEP associated magmatism and impact history of the Moon.Many zircons with different microstructures have been found in various petrographic settings in the regolith portion of SaU 169.They occur as(i)cogenetic crystals in the alkali anorthositic,felsitic,and quartz monzogabbroic rock clasts;(ii)crystals grown in impact melts;(iii)detrital zircons enclosed in impactites;and(iv)zircons embedded in the matrix.They have developed impact-related secondary microstructures of(i)granular texture,fine-grained texture,and solid-state recrystallization areas formed by post-shock high-temperature processes under>1673?,>800-1673?,and hundreds of?,respectively;(ii)lamellar reidite,a high-pressure polymorph of zircon with scheelite structure that transformed under>30 GPa.The reidite and fine-grained areas in zircon are likely formed in a young(<900 Ma),low-speed(average shock pressure<10 Gpa)impact event into lunar regolith,in which the former transformed from zircon in localized high?pressure regions and the latter produced in localized high?temperature regions.The two microstructures indicate that the shock pressure and temperature are very heterogeneous during the initial stage of impact into porous targets.Besides,some zircons have transformed into glass,which appears to be independent of impact events,instead,due to their high U concentrations that lead to severe radiation damage.The mechanism might also contribute to the formation of fine-grained regions in zircon.The ²⁰⁷Pb-²⁰⁶Pb ages of zircons in the regolith portion of SaU 169 range from?4.37 Ga to?3.86 Ga and peak at?4.32 Ga,?4.25 Ga,and?3.93 Ga,suggesting they have a similar source to the Apollo 14 lunar soil.The chronology of KREEP associated magmatism is found to be complicated,where felsitic magmatism started before?4.35Ga andcould have lasted for a few Ma,followed by quartz monzogabbroic magmatism(?4.22 Ga and?4.34 Ga),and finally,the alkali anorthositic magmatism(?3.88 Ga).The zircons in matrix span a similar chronological range to the others;however,they have lower U concentrations,likely indicating a source of different rock populations.The zircons in the regolith portion of SaU 169 have recorded multiple impact events,where those crystallized from impact melts and those have U-Pb systematics been completely reset(granular zircon aggregates and zircons with solid-state recrystallization rims)have ages mostly within a range of?3.90 Ga to?3.95 Ga,likely corresponding to the Imbrium impact event.Besides,some of these zircons have older ages(?4.34 Ga,?4.20 Ga,?4.07 Ga)that likely indicate pre-3.9 Ga impact events.