The evolution of lunar breccias : U-Pb geochronology of Ca-phosphates and zircon using Secondary Ion Mass Spectrometry

Abstract: Planetary bodies in our Solar System, including the Moon, were exposed to an intense asteroid bombardment between ~4.5-3.8 Ga, shaping their surfaces and leaving visible “footprints” in the form of large impact basins. The end of this period (~4.0-3.85 Ga), might have been marked by a cataclysmic increase in impacts, the so-called Late Heavy Bombardment (LHB), although this remains highly contentious. Since destructive processes, such as tectonics or erosion, have destroyed ancient (> 3.0 Ga) impact structures on Earth, studies of the early Solar System are mainly restricted to lunar samples, because impact structures are much better preserved on the Moon.In this thesis, we have therefore analysed impact breccias from three Apollo landing sites (Apollo 12, 14, and 17) with the overall aim to gain a better understanding of the lunar impact history. This endeavour included comprehensive textural and petrological analyses of the breccias and grains of interest (i.e. Ca-phosphates and zircon), as well as obtaining precise U-Pb Secondary Ion Mass Spectrometry (SIMS) ages. The U-Pb ages of Ca-phosphates obtained are consistent with the age of the Imbrium impact at ~3925 Ma, whereas an older age of ~3930 Ma yielded by Ca-phosphates in an Apollo 17 breccia might be linked to the formation of the Serenitatis basin. Furthermore, an impact event at ~3940 Ma was identified in zircon grains in Apollo 14 breccias, which is in agreement with older Ca-phosphate ages yielded in a previous study. The identification of three possible impact events within ~15 myr has important implications for the lunar bombardment history.However, there is a possibility that partial Pb loss from older grains during a relatively late event (e.g. Imbrium) might result in apparently older ages in Ca-phosphates. Incomplete resetting of the U-Pb system was recorded in zircon grains in an Apollo 12 breccia, leading to meaningless U-Pb ages which cannot be interpreted unambiguously as either magmatic or as impact events. Nevertheless, the U-Pb ages of several zircon grains occurring in lithic clasts in Apollo 14 breccias can plausibly be linked to magmatic activity, exhibiting several magmatic events between ~4286 Ma and ~4146 Ma. The data obtained in this thesis, together with previously published zircon and Ca-phosphate data, indicate several spikes in the magmatic and impact history during the first ~600 myr of lunar history. This study highlights the importance of combining high-precision age determination with thorough petrological and textural analyses in order to exclude meaningless ages and to interpret the impact and magmatic history of the Moon.