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

One of the outstanding questions in planetary science is the exact timing and duration of impact bombardment to the inner Solar System, including the purported Late Heavy Bombardment. Direct constraints on the timing and duration of the bombardment can be provided by absolute ages of the most ancient lunar surfaces and basins. A key issue is the timing of formation of the Serenitatis Basin, one of the largest and oldest lunar impact basins. Whether Serenitatis formed during a classical, spike-like Late Heavy Bombardment has been debated for decades; an ancient age (~4.1–4.3 Ga) would support a protracted bombardment and cast doubt on a narrow LHB spike. Although recent arguments have revived an ancient age for Serenitatis, no lunar sample had provided direct support. Most Apollo 17 impact-melt breccias initially thought to derive from Serenitatis have been reinterpreted as Imbrium products (~3.8–3.9 Ga). The Apollo 17 Station 8 intrusive highlands rocks (norites 78235 and 78236) are magnesian-suite cumulates emplaced very early in lunar crust formation (~4.3–4.4 Ga) and exhibit pervasive shock features, implying excavation from depth by a large basin-forming impact. Prior geochronology revealed a spread of ages (4.43–4.11 Ga) from various chronometers. Phosphate minerals (apatite and merrillite), sensitive impact chronometers due to moderate Pb diffusion closure temperatures and known shock response, offer a means to date impact events. This study presents in situ U-Pb and Pb-Pb ages of highly shocked phosphates in 78235 and 78236 via SIMS, complemented by Pb-Pb ages of baddeleyite to refine crystallization timing, atom-probe tomography to assess nanoscale structure and Pb distribution, and iSALE-2D impact simulations plus Kaguya MI remote sensing to evaluate provenance.

Prior studies have debated the lunar bombardment history, contrasting a spike-like Late Heavy Bombardment at ~3.9 Ga with a protracted impact flux extending earlier than 3.9 Ga. Serenitatis Basin’s age has been central to this debate; terrain and crater population analyses have suggested ancient ages (~4.1–4.3 Ga), but sample-based confirmation was lacking. Apollo 17 impact-melt breccias have largely been reassigned to the Imbrium event (~3.8–3.9 Ga) based on geochronology and terrain analyses, consistent with widespread ~3.8–3.9 Ga resetting in Apollo 14–17 samples and simulations predicting abundant Imbrium melt at those sites. Station 8 norites show extensive shock deformation and have yielded a range of isotopic ages (Sm-Nd, Rb-Sr, U-Pb, K-Ar/Ar-Ar) spanning 4.43–4.11 Ga, with more robust isochrons indicating crystallization ~4.33–4.34 Ga. SIMS Pb-Pb ages of accessory minerals suggest partial resetting around ~4.2 Ga in some grains. Ar-Ar ages near 4.19 Ga have been interpreted as thermal resetting yet not fully reset due to limited annealing features. The sensitivity of phosphates as impact chronometers has been established in Apollo breccias and terrestrial impact structures, and prior work documented shock microtextures in these phosphates. Remote sensing datasets (Kaguya MI) and crater chronology databases provide constraints on candidate young craters (e.g., Dawes) potentially responsible for later disturbances. Numerical impact models (iSALE) and regolith mixing studies inform ejecta transport, heating, and sample provenance over basin and crater scales.

• Samples and targets: Two thin sections (78235,43 and 78236,44) of Apollo 17 Station 8 norites were studied. Eight apatite, ten merrillite, and two baddeleyite grains were analyzed in situ.
• SIMS U-Pb and Pb-Pb analyses: Conducted on a CAMECA ims1280 at NordSIMS (Swedish Museum of Natural History). Sections were Au-coated (~30 nm). O2− primary beam (1.7–5.3 nA) with 7–10 µm spots and mass resolution ~5400 resolved molecular interferences. Data reduction used NordSIMS software and Isoplot 4.15. Common Pb (204Pb) was corrected using Stacey-Kramers present-day terrestrial composition. U/Pb calibration used the NW1 apatite standard (~1160 Ma). Matrix effects for merrillite (no dedicated standard) were assessed by separate plotting and inverse concordia of combined data, indicating negligible impact. 207Pb/206Pb ratios (post-common-Pb correction) provide robust ages for concordant grains. Baddeleyite Pb isotopes were analyzed with O2 beam, multicollection on four low-noise electron multipliers at M/ΔM ~4860, with oxygen flooding to enhance Pb yields (~7×). Detector gains were calibrated with BCR-2g; common-Pb-corrected 207Pb/206Pb assumed minor terrestrial contamination. Ages use Steiger and Jäger decay constants; uncertainties are 2σ.
• Atom-probe tomography (APT): Site-specific FIB-SEM preparation (TESCAN LYRA3) produced needle-shaped microtips from shocked apatite. Lift-out, Pt deposition, mounting to Si posts, annular milling, and low-kV final polish minimized Ga+ damage. Analyses used a CAMECA LEAP 4000X HR at 69 K, ultra-high vacuum (~10−11 Torr), UV laser (355 nm, 125–200 kHz, 250–400 pJ), maintaining ~0.006–0.008 ions/pulse. Time-of-flight mass spectrometry provided mass/charge; 3D reconstructions used IVAS 3.8.0 with voltage-curve-based tip radius evolution; SEM images constrained reconstruction for experiment M3. Mass resolving power ~800; spatial resolution ~nm-scale.
• Numerical impact simulations (iSALE-2D): Simulated formation of an 18 km Dawes-size crater with a 600 m dunite projectile at 17 km/s impacting a target comprising a 300 m mare basalt layer over dunite crust (representing Serenitatis peak ring material). Output tracked excavation depths, ejecta temperature and velocity fields during early excavation (e.g., 20 s post-impact). Ejecta reaching ~140 km was assessed, along with thermal exposure (<1000 K ~700 °C). Serenitatis basin formation was also modeled using established scaling, crustal thickness (30 km), and basin geometry to ensure consistency with GRAIL gravity.
• Remote sensing (Kaguya Multiband Imager, MI): Used 9 spectral bands (415–1550 nm), downsampled UVVIS to NIR resolution (~62 m/pixel). Topographic shading corrections applied (MAP 03 data). Radiative transfer modeling, constrained by FeO, estimated abundances of plagioclase, low-Ca pyroxene (orthopyroxene), high-Ca pyroxene (clinopyroxene), and olivine. Pixels matching norite compositions (allowing ±7 wt.% error) were searched within and around Dawes crater, and candidate spectra were fitted and compared to modeled spectra. GIS tools (IDL 5.5.3, ArcGIS 10.8.1) produced mineral maps.

• Baddeleyite Pb-Pb ages constrain norite crystallization at 4346 ± 18 Myr and 4323 ± 14 Myr, consistent with prior whole-rock/mineral isochrons (~4333 ± 59 Myr Sm-Nd/Pb-Pb) and accessory mineral ages indicating magmatic crystallization followed by later disturbance.
• Phosphate U-Pb data define an upper concordant intercept age of 4210 ± 14 Myr and a lower concordant intercept age of 504 ± 24 Myr (2σ), indicating two distinct Pb-loss events: a major impact at ~4.21 Ga and a younger disturbance at ~0.50 Ga.
• The ~4.21 Ga event aligns with bulk-rock 40Ar-39Ar age plateaus (4188 ± 13 Myr) and youngest accessory mineral ages, supporting a basin-scale impact that exhumed and pervasively shocked the norites, with peak pressures >50 GPa.
• APT reveals nanoscale polygonal grains (~10–20 nm and larger) in shocked apatite, with Mg-decorated grain boundaries and minor Pb trapped at boundaries, indicating shock-induced recrystallization and subsequent Pb mobility.
• Diffusion modeling for nm-scale grains shows that a short-duration thermal pulse (70 minutes to 1 day) must exceed ~550 °C to drive substantial Pb loss from ~10 nm grains, with ~700 °C needed for complete Pb loss in ~20 nm grains, consistent with the younger resetting at ~504 Myr.
• The younger ~0.50 Ga disturbance is not seen in other chronometers (e.g., Ar-Ar) and is best explained by a secondary, smaller impact heating event.
• Crater chronology and proximity analyses identify Dawes crater (18 km diameter, formation age ~454 Myr with 95% credible intervals encompassing 504 ± 24 Myr) as the closest suitable source. Kaguya MI data reveal at least three pixels on Dawes’ floor whose modeled mineral abundances match the Station 8 norites (plagioclase ~35–41 wt.%, orthopyroxene ~47–52 wt.%, clinopyroxene ~6–7 wt.%, olivine ~6–7 wt.), consistent with exposure of noritic material, likely in a central uplift.
• iSALE simulations show that ejecta derived from crust beneath the 300 m mare layer can be excavated from depths up to ~2 km, experience temperatures up to ~700 °C, and that ~20% of ballistic ejecta can travel ~140 km to the Apollo 17 site within ~30 s post-impact, matching thermal and transport constraints inferred from Pb diffusion.
• The norite boulder’s cosmic-ray exposure age (~260 Myr) indicates near-surface burial after emplacement and later exhumation/transport by a landslide, consistent with surface micrometeorite pitting and field observations.
• Absence of ~3.8–3.9 Ga resetting in these crustal norites argues against Imbrium as the major shock event; instead, the ~4.2 Ga impact recorded in phosphates is attributed to Serenitatis Basin formation.

The study targets the long-standing uncertainty in the timing of Serenitatis Basin formation and, by extension, the nature of early lunar bombardment. Concordant phosphate U-Pb ages and corroborating Ar-Ar ages demonstrate a major impact at ~4.21 Ga that pervasively shocked the Station 8 norites, while baddeleyite ages constrain prior magmatic crystallization at ~4.33–4.32 Ga. Microstructural and nanoscale evidence (shock-induced recrystallization and grain boundary Pb) directly link the phosphate ages to impact processes rather than igneous growth, supporting interpretation of a basin-forming event at ~4.2 Ga. The lack of any ~3.8–3.9 Ga signature in multiple sensitive chronometers excludes Imbrium as the causative event for these samples, strengthening attribution to Serenitatis. The younger ~0.5 Ga disturbance, absent in low-temperature chronometers, is best explained by short-duration heating during emplacement of ejecta from a proximal crater. Remote sensing and numerical modeling identify Dawes crater as a plausible source capable of excavating noritic material from beneath mare basalts, thermally exposing ejecta to ≤700 °C, and transporting it ~140 km to the Apollo 17 site. Together, these findings provide sample-based evidence that Serenitatis likely formed earlier than 3.9 Ga, implying either formation unrelated to the spike-like LHB or formation early in a protracted bombardment, thereby impacting lunar crater chronology calibrations.

This work provides microstructurally constrained, in situ phosphate U-Pb evidence for a ~4.21 Ga impact recorded in Apollo 17 Station 8 norites, consistent with Serenitatis Basin formation, and identifies a younger ~0.50 Ga thermal disturbance likely linked to Dawes crater ejecta emplacement. Baddeleyite Pb-Pb ages refine norite crystallization to ~4.33–4.32 Ga. Atom-probe tomography reveals nanoscale recrystallization and grain-boundary Pb consistent with shock resetting, while diffusion modeling and iSALE simulations reconcile thermal and transport requirements. Remote sensing confirms noritic materials exposed within Dawes. These results support an ancient (>3.9 Ga) age for Serenitatis and argue for reconsideration of early lunar crater chronology and impact flux histories. Future work should expand phosphate U-Pb and nanoscale analyses across diverse Apollo and lunar samples, integrate hyperspectral datasets where available to better constrain source lithologies, and employ coupled thermal-mechanical-ejecta modeling to refine ejecta survivability and boulder-scale thermal histories.

• The younger (~504 ± 24 Myr) Pb disturbance is not observed in other chronometers (e.g., Ar-Ar), implying subtle, short-duration heating that may be difficult to corroborate independently.
• Remote sensing constraints rely on MI multispectral data; lack of hyperspectral Spectral Profiler coverage over key pixels limits compositional confirmation.
• Merrilite U/Pb calibration uses an apatite standard; although tests suggest negligible matrix effects, residual uncertainties may persist.
• iSALE simulations cannot resolve the survivability and detailed thermal history of an individual ~0.5 m boulder; initial boulder size and fragmentation history are uncertain.
• The identification of Dawes as the source is plausible but not unique; similar orthopyroxene-plagioclase-rich compositions occur elsewhere, and ejecta pathways are complex.
• Field context indicates later surface processes (e.g., landslide at ~260 Myr) modified the boulder’s exposure history, adding complexity to provenance reconstruction.