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