LUNAR SAMPLE 60025: A STORY OF MULTIPLE LITHOLOGIES

The Moon is thought to have formed from a giant impact between the proto-Earth and a Mars-sized planetesimal named Theia. The debris from this collision then quickly coalesced to form the Moon. After accretion, the Moon then underwent a period of differentiation best explained by the lunar magma ocean (LMO) model, where the upper ~500 km or all of the Moon experienced melting and then subsequently cooled and differentiated. Apollo 16 lunar sample 60025 is thought to be a product of the LMO and one of the earliest examples we have of the primordial lunar crust, derived from the LMO. As a member of the ferroan anorthosite suite (FANs), 60025 has proved to be a challenge for geochronologists to precisely date in large part due to its low concentrations of relevant radiogenic isotopes. Despite this challenge, two separate studies have successfully yielded two distinct age dates for 60025 using the same Sm-Nd isotopic system (4.44±0.02 Ga and 4.367±0.011 Ga). These ages – while similar – are not within error of one another, so this raises the question as to how a single rock can yield two distinct crystallization ages. Using a mixture of petrographic, electron microprobe (EMP), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of five 60025 thin sections, this study provides valuable insight regarding the nature of this sample as a postulated flotation cumulate of the LMO and a sample of the early lunar crust. As a result of this comprehensive major, minor, and trace element analysis, this study offers an explanation for the discordant ages reported in the literature and the implications for future geochronologic studies of lunar crustal material. Preliminary data supports the conclusion that 60025 represents a mixed lithology of lunar material with each lithology corresponding to a different age.