Constraining the bombardment history of the Moon with a set of Apollo 14, 16 and 17 impact melt rocks

Abstract

One of the most important questions in lunar science is whether there was a lunar cataclysm – a sudden increase in the impactor flux around 3.9 Ga. The absolute ages of basin-scale impacts are the key to answering this question. Crystalline impact melt rocks from the Apollo landing sites can hold vital information about the lunar cataclysm, as they were potentially produced by such basin-scale impacts. This study aims to constrain (1) the number and nature of impact events represented in the Apollo collection of melt rocks, (2) the chemical and isotopic signatures of the source regions and impactors, and (3) the processes involved in the formation of a set of Apollo impact melt rocks. To achieve this, I combine analyses of major and trace element chemistry, highly siderophile element abundances, and the 87Rb-87Sr and 147Sm-143Nd isotopic systems, with 40Ar/39Ar dating of 35 crystalline impact melt rocks from the Apollo 14, 16 and 17 landing sites. To produce a coherent data set and minimise intrasample compositional variations, the different analyses were all obtained on the same split of each sample. The chemical data and petrographic information show that the melt rocks likely represent multiple impact events, some of which produced craters (<300 km diameter), whereas others were basin-forming events (>300 km diameter). The 40Ar/39Ar plateau ages obtained here reflect the crystallisation ages of the melt rocks, and thus offer a viable tool for dating the impacts that produced these deposits. Four different events (3950 ± 24 Ma, 3885 ± 8 Ma, 3815 ± 19 and Ma 3644 ± 42 Ma) are resolved using 40Ar/39Ar plateau ages from the Apollo 16 landing site, and one plateau age (3829 ± 15 Ma) is obtained for Apollo 14 sample 14310. The combination of chemical, isotopic and age data provide a strong case for a dominance of melt rocks related to a single basin-forming event, most likely the Imbrium, at 3885 ± 8 Ma. The chemical and isotopic data from the Apollo 17 impact melt rocks are consistent with an origin of these samples in the same event. The melt rocks associated with the other four resolved events are likely derived from smaller craters. However, uncertainty about the nature of these events is greatest for the youngest and oldest events. These findings weaken the case for a lunar cataclysm, which requires multiple impact basins to form around 3.9 Ga. The age and isotopic data confirm the notion that compositional variations within related groups of melt rocks are mainly the result of mixing of different lunar lithologies, reflecting heterogeneity in their crustal sources. However, in some cases the chemical and isotopic relationships might be better explained by igneous differentiation processes in an impact melt sheet. This study presents the first model that relates a set of impact-derived lunar whole rock compositions by an igneous process in an impact melt sheet.