Petrogenesis of magnesian troctolitic granulite clasts from Chang'e -5 drilling sample: Implications for the origin of ejecta material from lunar highlands

Abstract

<p>The lunar soil collected by the Chang'e-5 (CE-5) mission contains both local materials and components of various sources. The latter could have derived from the excavation of impact craters in the highlands away from the landing site, providing valuable information on the composition of the lunar crust beyond the mare basalts sampled by the CE-5 mission. In this study, we examined the mineralogy and petrology of magnesian troctolitic granulites (or troctolites) recovered in the CE-5 soil. The troctolites exhibit a granoblastic texture but have a finer grain size compared to Apollo granulites, and almost all plagioclase remains unaltered and not converted to maskelynite. The major element composition shows a SiO2 content of 45.4 wt%, CaO of 12 wt%, TiO2 of 0.2 wt%, Th of 0.1 ppm, and an Mg# of 76, lower than those of the Mg troctolitic lunar meteorites NWA 5744 and NWA 10401. The trace element budget indicates that the magnesian troctolites are KREEP-poor, which distinguishes them from the Mg-suite rocks in Apollo samples but is similar to some troctolitic lunar meteorites (NWA 10401). Furthermore, we predict –by using MELTS modeling– a plausible genetic connection between the Mg-suite rocks, and our troctolites, suggesting that the latter may be pristine and formed during a later stage along a fractional crystallization path. This scenario challenges the hybridization model and strengthens the evidence in favour of the fractional crystallization modeling. To ascertain the potential origin crater for the magnesian granulites, a comparative analysis was conducted with NWA 10401. Based on our findings, we propose that the Pythagoras crater, situated north of the CE-5 landing site, is the most probable source of the aforementioned magnesian troctolitic granulites. The ejecta from this crater may have been deposited between the Eratosthenian-aged mare basalts and the Imbrian-aged mare basalts beneath the CE-5 landing site. The KREEP-poor troctolitic granulite in the Chang'e-5 soil could represent paleo-ejecta from Pythagoras crater, excavated by subsequent impacts on the Eratosthenian-aged basalts and delivered them to the CE-5 site. The original location of the Mg troctolitic granulites is inferred to be near-surface, buried by the uppermost crustal rocks (<10 km), which is consistent with its contact with feldspathic impact glass.</p>