Unraveling the Components Within Apollo 16 Ferroan Anorthosite Suite Cataclastic Anorthosite Sample 60025: Implications for the Lunar Magma Ocean Model

• Published in: Journal of geophysical research. Planets 2022-02, Vol.127 (2), p.n/a
• Main Authors: Torcivia, M. A., Neal, C. R.
• Format: Article
• Language: English
• Subjects: Anorthosite; Breccia; brecciation; Chronology; Crystallization; Crystals; Derivation; Evolution; ferroan anorthosite; geochemistry; impact processes; Isotopes; Lunar crust; Lunar evolution; lunar magma ocean; Magma; Moon; Ocean models; Oceans; Plagioclase; Pyroxenes; Trace elements
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2020JE006799

The ferroan anorthosite suite (FAS) represents the only direct sampling of the lunar magma ocean (LMO) and potentially contains information on the earliest history of the Moon. Apollo 16 FAS sample 60025 is extremely important for understanding early lunar evolution, but unraveling this information is complicated. For example, 60025 has two distinct Sm‐Nd crystallization ages that in themselves encapsulate the complicated history of this sample, along with the cataclastic and heterogeneous textures exhibited. Here, we present new in situ major and trace element plagioclase and pyroxene data gathered from five thin sections of 60025 that highlight such complexities. Trace element data are used to derive equilibrium liquids and while many of the minerals analyzed here are consistent with derivation from the LMO, there are also a significant number of plagioclase and pyroxene crystals that crystallized from magmas inconsistent with current models of LMO evolution. Integration of Sm‐Nd isotopic data with the elemental data reported here indicate a nonchondritic LMO is possible and we confirm that 60025 is a polymict lunar breccia containing differently sourced material. Plain Language Summary The age of the lunar crust has previously been estimated by analyzing samples such as Apollo 16 Ferroan Anorthosite Suite (FAS) sample 60025. This sample and other members of this suite are considered to provide unique insight into the earliest evolution history of the Moon shortly after its formation. Constraining the ages of FAS samples has proven to be difficult and has yielded seemingly contradictory results with 60025 in particular, producing two distinct crystallization ages using the Sm‐Nd isotopic system. Here, we examine the individualcomponents of 60025 to better understand the formation history of this sample on a mineral to mineral scale. Using in situ analysis techniques and applying these data to models of lunar evolution, we have determined that this sample represents a mixture of rock types, some consistent with derivation from the LMO, while others are not. Combined with its multiple reported literature ages, we have determined that this sample better represents a lunar breccia rather than a pristine sample of the original lunar crust as previously thought. As such, interpretations of any ages reported from analysis of this sample require careful consideration. Key Points Textures of ferroan anorthosites returned by Apollo display intense post‐crystalliza