A1 - Chronometric investigations of ancient lunar impact rocks

Research areas: Cosmochemistry, geochemistry


Lunar impact basins and their deposits represent the most detailed and best-preserved record of the timing, flux, and composition of late accretion in the inner solar system. However, reconciling interpretations of isotopic ages (suggesting a late heavy bombardment at 3.9 Ga) with the cratering record remains controversial. Thus, applying improved methods and novel approaches to the geochronology of ancient (>3.8 Ga) lunar impact rocks will produce new constraints for models of the lunar accretion history between 4.5 and 3.8 Ga. New Lu-Hf, Sm-Nd, Rb-Sr, and Pb-Pb ages and applying in situ U-Pb dating of zircons in lunar impactites by ion microprobe will provide new insight into the significance of ages of impact events and isochron resetting due to late basin-forming impacts. In particular the U-Pb zircon and Lu-Hf systems are characterized by high closure temperatures, and will likely display a different response to secondary overprints by later impacts compared to the Ar-Ar plagioclase chronometer. The samples we will analyse have siderophile element compositions that may be related to specific impactors, include samples that have yielded ambiguous Ar-Ar dates, or apparently predate the ~3.9 Ga distribution peak that is dominated by Ar-Ar ages. Linking the new data to impact rocks with specific highly siderophile element (HSE) and siderophile volatile element (SVE) compositions (subproject B1) may also place constraints on the flux of volatile rich planetesimals during late accretion. Another objective will be to provide new data to improve the chronology function of the lunar cratering record, in particular for older geologic units that comprise ejecta deposits. The lunar chronology function, established in the early 1980s, provides the basis for ages of other bodies of the inner solar system that were determined by crater counting. Some ages used for the chronology function were obtained on lunar impactites from the Apollo 16 landing site. However, the significance of many of the ages obtained in the 1970s is unclear. Thus, here we will use multiple decay systems to date petrographically well-characterized impact melt rocks and breccias, both simple and complex. By offering updated constraints on the lunar accretion history, this new data will then be used to test different models of post-4.5 Ga mass accretion rates in the inner solar system (A2, A3).