The Moon's surface bears the scars of intense early bombardment by asteroids, comets, and planetesimals. Understanding the timing and sequence of these impact events, particularly the formation of large basins (>150 km), is key to unraveling the dynamics of the early Solar System. The South Pole-Aitken (SPA) basin, the largest and oldest visible impact basin in the inner Solar System, holds a pivotal position in this understanding. Its formation age marks the onset of the lunar basin-forming epoch. Previous dating efforts have yielded inconsistent results, leaving the SPA basin's formation age poorly constrained. This study aims to refine this age by analyzing the lunar meteorite Northwest Africa (NWA) 2995, which has been suggested to originate from the SPA basin based on its chemical composition. Precisely dating SPA basin formation is essential because the event is believed to have excavated material from the lunar lower crust and upper mantle, creating a significant melt pile that differentiated into various igneous rocks. The impact's influence extended beyond the immediate basin, possibly impacting the lunar interior and causing large-scale instability, including triggering partial melting on the Moon's nearside. Recent missions, such as Chang'e-6, have collected samples from the SPA basin, providing additional opportunities to constrain its formation age, while the possibility of Earth-crossing orbits for SPA material suggests lunar meteorites like NWA 2995 could provide valuable chronological information.

Studies on lunar basin formation have utilized various techniques to determine the ages and relative order of these events. These include crater counting models, which analyze the density and size distribution of impact craters on the lunar surface to infer surface ages. However, these techniques often produce ambiguous results for the oldest basins due to factors such as saturation and overlapping events. Radiometric dating of lunar samples, primarily using the U-Pb system in zircon and other minerals, provides a more precise approach. This has provided information about the ages of various lunar basins, including Serenitatis, Nectaris, Crisium and Imbrium. However, the age of the SPA basin remains controversial with various dating estimates using crater counting and/or radiometric dating. Previous attempts to link meteorites to specific lunar basins have often relied on compositional similarities between the meteorite and surface materials, inferred from remote sensing data. The results have also been controversial for older basins that lack younger overprint. This paper employs a multi-faceted approach, combining geochemical analyses with high-precision radiometric dating of a candidate SPA-derived meteorite.

The research focused on the lunar meteorite Northwest Africa (NWA) 2995, a breccia with a diverse range of clasts. The methodology involved a detailed petrographic analysis using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) to map elemental distributions and identify various components, including feldspathic impact melt breccias, anorthositic gabbronorites, mafic clasts, granulitic breccias, pyroxene, granophyres, glass fragments, and mineral fragments. Cathodoluminescence (CL) imaging was used to examine the internal textures of mineral grains, particularly zircon. Electron backscatter diffraction (EBSD) analysis was conducted to study the crystal structure and strain levels within zircon grains. Secondary ion mass spectrometry (SIMS) was used for high-precision radiometric dating of various phases. Ca-phosphates (apatite and merrillite) and zircon were dated using U-Pb and Pb/Pb systems. Pb/Pb dating was also conducted on silicate minerals (K-feldspar, pyroxene, silica, and apatite) within granophyre clasts. A chi-squared analysis compared the bulk rock composition of NWA 2995 with the Lunar Prospector gamma-ray dataset to identify the most likely launch sites on the lunar surface. Raman spectroscopy was used to characterize the structure of silica phases within the meteorite. The data was processed and analysed using multiple software such as ImageJ, Adobe Photoshop, LabSpec 6, Isoplot, and in-house developed software. In addition, various standards such as NW-1 apatite and Phalaborwa baddeleyite were used to calibrate the U-Pb fractionation and common Pb contributions were corrected using the Stacey and Kramers model for present-day terrestrial Pb isotopic compositions.

Petrographic analysis of NWA 2995 revealed a complex breccia with a variety of clasts, indicating a complex geological history. The mineral chemistry supported a lunar origin. The majority of clasts showed evidence of brittle deformation and shock pressures ranging from unshocked to moderately shocked. The presence of glass spherules confirmed the regolith breccia nature. SIMS dating of Ca-phosphates yielded an upper intercept age of 4,317 ± 12 Ma, while zircon dating produced an upper intercept age of 4,330.6 ± 3.5 Ma. Pb-Pb dating of minerals in granophyre clasts gave an age of 4,317 ± 12 Ma. The consistent ages from different mineral phases and dating techniques strongly point to a major geological event at ~4.32–4.33 Ga. The geochemical analysis showed that NWA 2995 is most similar in composition to regolith in the central SPA basin on the lunar farside, specifically within the central depression zone thought to represent the basin's differentiated impact melt pile. Raman spectroscopy analyses revealed evidence of radiation damage in some zircon grains, consistent with a long residence time near the lunar surface. No evidence of high pressure phases was seen. The results thus indicate a formation age of 4.32-4.33 billion years for the SPA basin, older than previously suggested for Serenitatis, Nectaris and Crisium basins. Importantly, the lack of an Imbrium basin overprint (3.92 Ga) also confirms the basin predates this event.

The 4.32–4.33 Ga age for the SPA basin formation, derived from the analysis of NWA 2995, pushes back the timeline of the early lunar bombardment. This age is consistent with the older end of the estimates derived from crater-counting models. It strongly suggests that the SPA basin-forming event was unique, preceding the main period of lunar basin formation by about 120 million years. This finding supports an accretion tail-end or saw-tooth impact model, rather than a terminal cataclysm model, implying that lunar bombardment was not concentrated in a short period. The age also implies that older basins, potentially forming before ~4.33 Ga, were erased. This is likely due to massive resurfacing caused by the SPA impact itself or other processes such as high early lunar heat flow. The composition of NWA 2995 is particularly useful since it is sourced from a region of the lunar farside with a Pre-Nectarian age, consistent with the obtained radiometric dates.

This study provides compelling evidence for a 4.32–4.33 billion-year age for the Moon's South Pole-Aitken basin, based on the analysis of the lunar meteorite NWA 2995. This significantly advances our understanding of the early lunar bombardment history, supporting models with a more protracted and less intense late heavy bombardment period. Future missions that collect samples from the SPA basin will be able to further corroborate this result. Future research could focus on analyzing more samples from the SPA basin, employing multiple dating techniques and geochemical analysis to further constrain the age and explore the impact's implications for lunar evolution and early Solar System processes.

The study relies on the interpretation that NWA 2995 is indeed from the SPA basin. Although the compositional and isotopic data strongly support this origin, it is possible that there could be areas with similar compositions elsewhere on the Moon that have not been mapped. Furthermore, the radiometric dates represent the time of a major resetting event, either crystallization or impact reset, rather than the precise time of impact. The potential impact of terrestrial contamination on some of the analyses is also noted, which could potentially cause some uncertainties.