Paper No. 239-4
Presentation Time: 9:00 AM
FERROAN ANORTHOSITE 60025: A HETEROGENEOUS SAMPLE OF THE LUNAR CRUST
The lunar magma ocean (LMO) model has served as the established mechanism for the differentiation of the Moon since the first samples of lunar crustal material were returned by Apollo 11. However, a process of serial magmatism has also been proposed as an alternative mechanism to create the anorthositic crust. In either case, analysis of the ferroan anorthosite suite (FANs) of lunar material has served as a primary line of evidence for both leading hypotheses. Sample 60025 is a large FAN sample that has been precisely age dated twice using the Sm-Nd isotopic system, yet each separate geochronologic study yielded a distinct age (4.44±0.02 Ga and 4.367±0.011 Ga). The younger age especially calls into question the LMO model by necessitating the extension of the LMO beyond what most thermal models would allow. Despite previous characterizations of 60025 as a pristine sample of the lunar crust, this study of 60025 demonstrates its heterogeneous nature through a comprehensive petrographic, major, minor, and trace element analysis of 5 separate thin sections representing a diverse sampling of the parent sample. The data gathered by this study from electron microprobe and laser ablation ICP-MS and the subsequent calculation of corresponding equilibrium liquids has led us to the conclusion that 60025 is in fact a polymict lunar cataclasite containing multiple distinct parent lithologies – some of which are inconsistent with derivation from the LMO – and that the two discordant ages reported in the literature are a reflection of its heterogenous nature. We caution that future analyses of FANs must be cognizant of the possibility of the presence of multiple non-cogenetic phases within lunar samples, even for those samples which have previously been given a pristine label.