NEW INFORMATION FROM OLD LUNAR ZIRCON – INTEGRATING U-PB AND (U-TH)/HE CHRONOMETRY

Zircon has provided important insights into the evolution of the Moon. Resilience to U-Pb resetting has allowed age preservation through the intense effects of bombardment to record the timing of processes such as lunar crust formation, while still also recording basin-scale impacts. Employing lunar zircon as a low-temperature (U-Th)/He thermochronometer has widened the thermal window that can be recorded. The closure temperature to diffusion of ⁴He from zircon depends on accumulated radiation damage to the crystal structure, meaning more damaged grains (higher U+Th) are susceptible to He loss during lower temperature thermal events. Hence, when integrated, the high-T U-Pb and low-T (U-Th)/He systems in zircon we have the potential to access the protracted record of impacts and other events that have shaped the Moon and the inner solar system.

Here we present new research on lunar impact-melt breccia sample 14311, interpreted to have formed in the Imbrium impact at ca. 3.95 Ga, which integrates U-Pb zircon ages of pre-impact lunar crust with (U-Th)/He (ZHe) thermochronometry from the same grains. U-Pb ages document evidence for three high-T events in the sample history: (1) formation of ca. 4330 Ma crust; (2) igneous activity or impact-melt sheet crystallization at ca. 4250 Ma; and, (3) ca. 3950 Ma impact-shocked zircon and zircon crystallized from impact melt (Hopkins & Mojzsis, 2015). ZHe dates range from ca. 3950-3500 Ma at low eU (<100 ppm, eU = U + 0.235*Th < 100 ppm), dropping to a consistent population of younger dates at higher eU. The date-eU correlation is typical of that observed in normal terrestrial zircon samples. Thermal history modeling of the lunar ZHe data set indicates a best-fit event history dominated by two thermal events, corresponding to the age of Imbrium impact (ca. 3.95 Ga) and a second event at ca. 100 Ma.

The results from this study underscore the utility of low-T thermochronometers in ancient samples and show that zircon grains with long histories of accumulated radiation damage can retain He over ~4 Gyr timescales and still yield meaningful ZHe dates. Further, integrating U-Pb and ZHe from the same grains allow us to more comprehensively document the timing and extent of impacts on the moon and throughout the inner solar system.