The Early Lunar Magmatic and Impact Histories Recorded by Apollo Zircons
The nature of the early lunar magmatic and impact histories remains a fundamental, yet poorly understood area of planetary science. Lunar zircons are thought to have formed in enriched residual melts and their crystallization ages have thus been used to infer the timing of KREEP magmatism. However, models of the lunar magma ocean (LMO) solidification are inconsistent with the implied duration of KREEP magmas (4.4-3.9 Ga). Additionally, the proposed late heavy bombardment (LHB), or lunar cataclysm, during a similar era of lunar history has recently been called into question. The latter has implications beyond the Moon, because the lunar impact record is used to constrain dynamical models of solar system evolution and to calibrate ages of planetary surfaces. Investigating early lunar history requires samples older than the proposed cataclysm, which may also record signatures of magmatic crystallization and subsequent impact events. Lunar zircons are ideal for this study for several reasons: first, the population of lunar zircons identified in Apollo sample thin sections all have 207Pb-206Pb crystallization ages older than the LHB, and second, zircons from both Earth and the Moon have been shown to retain textural and chemical evidence of impact shock events.
The research presented in this thesis consists of coordinated analyses of individual zircons recovered from Apollo 14, 15, and 17 samples aimed at investigating both facets of the early history of the Moon. An extensive U-Pb and 207Pb-206Pb age survey reveals that >90% of lunar zircons crystallized before ~4.1 or 4.2 Ga. The relative scarcity of textures commonly associated with Pb-loss and age disturbances in the studied zircon population indicates that these ages likely reflect igneous crystallization ages. This suggests a 200 – 300 Myr duration of KREEP magmatism, which is shorter than previous estimates based on the full range of lunar zircon ages, but is more consistent with the time scales for LMO solidification models.
Trace element analyses reveal that contributions from micron to sub-micron impact melt
glass inclusions common in lunar zircons are significant sources of contamination. After
filtering the data to remove analyses that contained these inclusions, the lunar zircons can be described by one Rare Earth Elements (REE) pattern that is characterized by a negative Eu anomaly and an absence of a positive Ce anomaly. This observation is contrary to previous reported results that did not account for contamination effects, and suggests that the lunar zircons formed in a reducing environment. Concentrations of titanium were measured to calculate crystallization temperatures. Using new estimates for oxide activities, we find that temperatures range between 958 ± 57° to 1321 ± 100°C. This range is bounded by the dry granite solidus and zircon saturation temperatures for KREEP rich-samples, and do not require hydrous conditions as suggested by previous authors (who assumed equal oxide activities). The zircon trace element analyses reported herein therefore support the general understanding of the lunar environment as reducing and anhydrous.
We also present the first U-Pu-Xe age analyses of lunar zircons and compare them with terrestrial shocked zircons from the Vredefort impact structure in South Africa. The terrestrial zircons show no evidence of impact-associated Xe-loss, but do record a signature of a later local intrusive event. This suggests impact shock alone does not result in appreciable Xe-loss, however prolonged exposure to elevated temperatures can disturb U-Xe ages. Xe analyses of the lunar zircons reveal that some grains retain their fission Xe for >3.8 Ga, while others appear to have experienced significant Xe-loss within the last ~0.5 Gyr. Contributions from an isotopically heavy Xe component, most likely due to spallation from REE, allowed for only minimum degassing ages to be determined. Nonetheless, the high retentivity of Xe, and consequently old U-Xe ages, in some lunar zircon grains suggests that U-Pu-Xe dating is a promising method for investigating the early impact history of the Moon, and future analyses to constrain spallation Xe yields will allow for more precise degassing ages to be determined.