Mineralogy of Yamato 983885 lunar polymict breccia with a KREEP basalt,a high-Al basalt, a very low-Ti basalt and Mg-rich rocks

摘要

Y983885 is a polymict regolith breccia with a KREEP basalt, Mg-rich troctolite/norite, a high-Al basalt, a very low-Ti basalt, a granulite originated from ferroan anorthosite, and Si, Na-rich impact spherules. An igneous KREEP basalt is first reported among lunar meteorites to date. The KREEP basalt is mineralogically distinct from Apollo KREEP basalts due to the lack of the typical Ca zoning from orthopyroxene to pigeonite, instead, the presence of the co-existing pigeonite/augite with chemical zonings and micron-scale exsolution. With these mineral characteristics, the KREEP basalt is probably cooled slightly slower than the Apollo KREEP basalts under the subsurface condition such as hypabyssal setting or lava pond. Further study of the additional samples is necessary to fully understand the petrogenesis of this new KREEP basalt. The troctolite and norite are also distinct in lower mg# of mafic minerals and higher modal abundance of olivine in norite, comparing to Apollo troctolites and norites, implying the existence of a rock type with intermediate modal abundance between norite and troctolite, and the compositional diversity of Mg-rich lithologies. Simultaneous occurrence of a KREEP basalt and a genetically KREEP-related, high-Al basalt, a Mg-rich troctolite/norite and the Si, Na-rich impact glasses can constrain the source region of Y983885 to the KREEP-rich Procellarum terrane in the northwestern hemisphere of the lunar nearside.