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

Impact cratering on the Moon and the associated size-frequency distribution of impact craters has been used to determine the ages of the lunar and planetary surfaces across the Solar System1,2. Radiometric age-dating of lunar samples plays a critical role in this endeavour3. However, due to paucity of pristine samples and challenges associated with interpreting the isotopic data, the lunar crater-chronology function remains poorly constrained above 3.9 billion years (Gyr), and with only a handful of craters with ages < 1 Gyr4. Additionally, much of the timescale for the earliest lunar cratering history hinges on the formation age of the Serenitatis basin – a topic of vigorous debate5–9. Here we report on U-Pb isotopic data and nanoscale observation of phosphate minerals within shocked lunar norites from the Apollo 17 Station 8 boulder that record a discordant array between an older event at ~4.2 Gyr, and a younger disturbance at ~0.5 Gyr. Based on available lunar cratering records and new impact modelling, we propose that the Station 8 boulder was likely ejected when the Dawes crater, located on the southeastern rim of the Serenitatis basin, formed ~0.5 Gyr ago10. Formation of the Dawes crater excavated the highly-shocked noritic basement, which had also recorded a ~4.2 Gyr impact event – an age we ascribe to the formation of the Serenitatis basin. Implications of ancient (pre-Nectarian), ~4.2 Gyr age of the Serenitatis basin are far-reaching, challenging the paradigm of the Late Heavy Bombardment and setting important constraints on the older segment of the crater size-frequency function.