ROCKS IN LUNAR REGOLITH: SMALL IS BEAUTIFUL, REDUX

“Gifts that keep on giving” - This simple phrase is oft used by Dr. Charles Shearer in reference to Apollo samples. The statement is especially apropos for the small fragments of rock that occur in Apollo regolith. One such rocklet is 12032,366-18, a 41 mg fragment of basalt from Apollo 12 that is unusually rich in incompatible trace elements, including thorium (~7 ppm). The composition is distinct from the main groups of basalt at this and other Apollo landing sites; we infer that 12032,366-18 is a bit of ejecta from an impact crater somewhere in the region. We previously analyzed the sample by instrumental neutron activation analysis (INAA), with included flux monitors so we could analyze Ar(40/39) isotopes. Following INAA, we made a polished probe mount and used remaining fragments for Ar analysis. Two separate splits yielded disturbance ages of 688+/-10 Ma and 487-617 Ma, and crystallization >2.3 Ga (Barra et al., 2006, GCA 70, 6016-6031). The disturbance age is distinct from, and younger than, that of Copernicus, whose ejecta are prominent at the Apollo 12 site and yield a disturbance age ~800 Ma. Subsequently, we conducted crystallization experiments using the sample’s bulk composition. Stadermann et al., (2022, MAPS 57, 794-816) showed that the bulk composition is representative of the melt from which this rock crystallized and that its Si-K-rich mesostasis formed from late-stage silicate-liquid immiscibility. More recently, we analyzed the polished mount using SIMS to measure its Pb isotopes and determine its crystallization age (Deligny et al., 2024, European Lunar Symposium). The age indicated by Pb-Pb chronology is 3400 +/- 16 Ma. Using a combination of chemical composition (FeO ~20 wt.%, TiO₂ ~4 wt.%, Th ~7 ppm) and age information, we infer that this basalt fragment likely originated in the vicinity of Kepler crater for which crater-count ages indicate basaltic surfaces in the range 3.0-3.6 Ga. From the Ar-Ar data, we infer that Kepler crater formed in the range of 500-700 Ma, with ejection to the Apollo 12 site creating the disturbance. This age for Kepler is in family with estimates based on crater counts. Thorium enrichment may be characteristic of the expansive basalts of the Procellarum region of the Moon, as also sampled by the ~2 Ga Chang’e-5 basalts, and possibly associated with Th-enrichment of the lunar nearside in the Procellarum KREEP Terrane.