Lunar samples record an impact 4.2 billion years ago that may have formed the Serenitatis Basin

One of the significant unanswered questions in planetary science is the precise timing and duration of impact bombardment in the inner Solar System, including the proposed Late Heavy Bombardment (LHB). Direct constraints on this bombardment can be obtained from the absolute ages of the most ancient lunar surfaces and basins. Closely related is the timing of the Serenitatis Basin's formation—one of the Moon's largest and oldest impact basins. Whether Serenitatis formed during the classical LHB has been debated for decades. An ancient age for Serenitatis would support a protracted bombardment period, challenging the notion of a classical, spike-like LHB. Recent arguments have revived interpretations of its ancient (~4.1–4.3 billion years, Gyr) age, but direct supporting evidence from lunar samples has been lacking. Most Apollo 17 samples are impact-melt breccias initially thought to originate from the Serenitatis Basin, but later studies linked them to Imbrium formation (~3.8–3.9 Gyr). The Apollo 17 samples 78235 and 78236, magnesian-suite norites, are among the oldest components of the lunar crust. Emplaced early during lunar crust formation (~4.3–4.4 Gyr), they were sampled at Station 8 in Taurus-Littrow Valley, at the Serenitatis Basin's edge. Their complex impact history and transportation are reflected in their geochronology, with ages spanning 4.43 to 4.11 Gyr. Phosphate minerals (apatite and merrillite) are highly sensitive impact chronometers due to their moderate closure temperature and response to shock metamorphism. This study presents the first in situ U-Pb and Pb-Pb ages of highly shocked apatite and merrillite in 78235 and 78236, complemented by new Pb-Pb ages of baddeleyite, using secondary ion mass spectrometry (SIMS) and atom-probe tomography (APT) to analyze nanostructure and chemical composition. Impact simulations and remote sensing data are used to investigate the samples' provenance.

Numerous studies have attempted to pinpoint the timing and duration of the Late Heavy Bombardment (LHB), a period of intense asteroid impacts in the early Solar System. Determining the age of major lunar basins, such as Serenitatis, is crucial to understanding this period. Previous research on the Serenitatis Basin's age has yielded conflicting results, with some suggesting an age around 4.1-4.3 billion years, and others placing it closer to the generally accepted age of Imbrium at 3.8-3.9 billion years. This discrepancy stems from challenges in dating the oldest lunar surfaces and the complex impact history recorded in lunar samples. Studies employing various geochronometers (Rb-Sr, Sm-Nd, K-Ar, U-Pb, 40Ar-39Ar) on Apollo samples have provided age estimates for lunar crust formation and subsequent impact events, but often with significant uncertainties and discrepancies. Previous studies have investigated the Apollo 17 samples and their associated impact events, but have not yielded conclusive evidence regarding the Serenitatis Basin's formation age. The use of phosphate minerals, due to their sensitivity to impact events, is a relatively recent approach to resolving these ambiguities.

This study employed several techniques to determine the age of impact events affecting the Apollo 17 samples. **Secondary Ion Mass Spectrometry (SIMS):** U-Pb and Pb-Pb isotopic measurements were performed using a CAMECA ims1280 ion microprobe on apatite, merrillite, and baddeleyite grains from samples 78235 and 78236. A duoplasmatron-generated primary beam of O2 ions was used for spot analyses. Data reduction involved correcting for terrestrial contamination using the Stacey and Kramers model. Sample Pb/U ratios were calibrated against the NW1 apatite standard. Decay constants from Steiger and Jäger were used for age calculations. The methodology carefully considered and accounted for potential matrix effects on U/Pb ratios, particularly for merrillite analyses. **Atom-Probe Tomography (APT):** Three-dimensional compositional and spatial imaging was performed using a CAMECA local-electrode atom probe (LEAP 4000X HR) on FIB-prepared apatite microtips. This technique enabled nanoscale resolution of the apatite's structure and chemical composition, including the distribution of lead (Pb), allowing for the identification of grain boundary migration and recrystallization related to shock events. Data processing and peak ranging were done using CAMECA's IVAS software. **Numerical Impact Simulation:** The iSALE-2D shock physics hydrocode was used to simulate the formation of both the Serenitatis Basin and the Dawes crater, providing insights into the dynamics of impact events and the resulting ejection of material. The model parameters for Serenitatis were based on previous studies. The simulation focused on determining the temperatures experienced by ejecta, specifically from material beneath mare basalt, in the formation of the Dawes crater. **Remote Sensing:** Kaguya Multiband Imager (MI) data were used to analyze the mineral composition of the Dawes crater floor, walls, and surrounding area. This involved radiative transfer modeling of the four major lunar minerals (plagioclase, olivine, low-Ca pyroxene, and high-Ca pyroxene) to match the mineral composition of the Apollo 17 norites.

The study yielded several key findings: 1. **Baddeleyite Pb-Pb ages:** The analysis of baddeleyite grains yielded ages of 4346 ± 18 and 4323 ± 14 Myr, consistent with previously reported crystallisation ages of the norites. 2. **Phosphate U-Pb and Pb-Pb ages:** The shocked phosphates revealed two significant Pb loss events: an upper concordant intercept of 4210 ± 14 Myr, and a lower intercept of 504 ± 24 Myr. The older age is younger than the crystallisation age, indicating Pb loss approximately 130 Myr after crystallisation, consistent with a major shock event. The younger age suggests a secondary disturbance. 3. **Nanoscale observations (APT):** APT analysis of the shocked apatite revealed a fine-grained nanostructure (~10 nm and larger polygonal grains) indicating shock-induced recrystallisation. This recrystallization facilitated rapid Pb diffusion, explaining the significant Pb loss recorded in the 504 Myr event. 4. **Dawes crater as a possible source:** Modeling suggests that the 504 Myr event could have been caused by a secondary impact from the Dawes crater, located approximately 140 km from the Apollo 17 landing site. Kaguya MI data revealed a close match in mineral composition between the Apollo 17 norites and material found on the Dawes crater floor. iSALE simulations support the possibility of ballistic transport of ejecta from the Dawes impact to the Apollo 17 site, with temperatures consistent with the observed Pb diffusion. 5. **Serenitatis Basin formation age:** The study links the 4210 ± 14 Myr impact event recorded in the phosphates to the formation of the Serenitatis Basin, suggesting a formation age older than the generally accepted Imbrium age. This is supported by the lack of evidence for a ~3.8-3.9 Gyr impact event in the studied samples.

The findings challenge the prevailing view of a concentrated Late Heavy Bombardment, suggesting that the Serenitatis Basin formed either before or in the early stages of this period. The precise age of 4.2 billion years for the Serenitatis Basin formation is a significant finding, as it pushes the timing of major lunar basin formation further back in time. The successful use of phosphate minerals as sensitive impact chronometers opens new avenues for studying impact events on other planetary bodies. The identification of the Dawes crater as a potential source for the Apollo 17 samples highlights the importance of considering secondary impact events when interpreting the complex geological history of lunar samples. The detailed mineralogical and textural analyses, along with advanced modeling techniques, provide robust evidence for the proposed interpretation, although uncertainties remain concerning the exact size and trajectory of the boulder throughout its history. The study's findings support a protracted, rather than a spike-like, early bombardment history of the Moon.

This research provides compelling evidence for the 4.2 billion-year-old age of the Serenitatis Basin, significantly predating the commonly accepted age of the Imbrium Basin. The study successfully utilizes phosphate minerals as highly sensitive impact chronometers and employs state-of-the-art techniques, including APT, numerical modeling, and remote sensing data analysis. This work suggests that major lunar basin formation extended further into the early history of the Solar System than previously thought, challenging existing models of the Late Heavy Bombardment. Future research should focus on expanding the use of phosphate geochronology to other lunar and planetary samples to refine our understanding of early Solar System bombardment.

The study's interpretations rely on the assumption of a single major shock event at 4.2 billion years ago for the Serenitatis Basin formation. Although the evidence strongly supports this, other less likely scenarios cannot be entirely ruled out. Furthermore, the precise ejection and transport history of the boulder remains uncertain. While the Dawes crater is a strong candidate source, other nearby craters cannot be completely discounted. The use of apatite as a chronometer is relatively new, and further research is needed to fully validate its reliability across a wider range of geological conditions and impact intensities.