Reconstructing Early Earth and Lunar History: Insights from Terrestrial and Extraterrestrial Zircon

Zircon (ZrSiO4) is a common accessory mineral in terrestrial and extraterrestrial rocks, valued for its ability to record magmatic conditions and serve as a geochronometer. Its resilience to weathering, capacity to incorporate U and Th while excluding Pb during growth, and ability to retain REEs, Hf, and Ti make zircon a powerful tool for probing high-temperature events such as meteor impacts. These properties have provided insights into early Earth’s evolution, from the Hadean era to the emergence of an atmosphere, oceans, felsic crust, plate tectonics, and the ingredients for life. However, Hadean zircon are rare, limiting data and necessitating modern analogs to improve our understanding. This dissertation comprises three parts: 1) geochemical characterization of inclusions in impact-formed zircon, 2) mathematical modeling of the size-frequency-distribution of impactors during the Hadean, and 3) tomographic analysis of Pb nano-spheres in lunar zircon. First, mineral inclusions in zircon from Sudbury, Canada, and Morokweng, South Africa, are compared to possible parent rock compositions. Inclusions trace host rock composition, and trends in Total Alkali Silica and Quartz – Alkali Feldspar – Plagioclase – Feldspathoid diagrams suggest Hadean zircon formed in tonalite-trondhjemite-granodiorite-like rocks. This research reconstructs ancient crustal compositions erased from the geologic record. The second part of this treatise models of high-energy impacts revealing that SiO2 enrichment occurred early in Earth’s history. Geochemical fractionation patterns in Sudbury’s differentiated impact melt sheet suggest impacts contributed to felsic proto-continents, with a Late Heavy Bombardment producing magma with ≥58 wt.% SiO2, covering ca.10% of Earth’s surface to a minimum thickness of approximately 2 km. Lastly, the third part of this dissertation uses the isotopic and trace element composition of extraterrestrial zircon in order to evaluate high temperature impact events. Atom Probe Tomography (APT) lattice reconstructions for Apollo 14 lunar zircon, identify Pb nano-clusters linked to high-energy impacts. The APT reconstruction of a Pb nanocluster within a ~4.3 Ga lunar zircon has a 207Pb/206Pb ratio of 1.39 ±1σ indicating Pb accumulation from 4.3–3.9 Ga, reflecting a major lunar impact event at ~3.9 Ga. This suggests the Moon’s surface experienced a high-energy event, or events, at 3.9 Ga.