Inclusions in impact-formed zircon as a tracer of target rock lithology: Implications for Hadean continental crust composition and abundance

Abstract

Impacts play an essential role in the formation and possible habitability on early Earth and are the dominant mechanism controlling surface geomorphology on many solar system bodies. Without a significant >4 Ga rock record, geochemical variations and micron-scale inclusions within detrital Hadean zircon (Earth's oldest material - primarily from the Jack Hills, Western Australia) have provided valuable insights into the near-surface conditions of this crucial period in Earth's evolution. We report primary inclusion mineralogy of impact-formed zircon and the host impact melt from the Sudbury and Morokweng impact structures with estimates of target rock lithology to probe the composition of Earth's earliest crust given the possibility that Hadean grains were created in ancient impacts. Applying the theory of uniformitarianism between preserved and ancient impactites and utilizing geochemical proxies, we highlight the compositional evolution of an impact melt from a given target rock and the associated primary inclusion assemblages. Whole-rock X-ray Fluorescence and thick-section analyses coupled with primary inclusion populations from known terrestrial impact-formed zircon yield proportional mineralogy of the impact melt sheet from which they crystallized. Additionally, target rock lithology consistently plots within an area on a Total Alkali Silica (TAS) diagram centered amongst the granophyre and noritic units of an impact melt. Deducing similar trends of the impact melt mineralogy combined with TAS and Quartz-Alkali-Plagioclase ternary diagram melt pathways, we infer that if Hadean zircons from Jack Hills were impact-formed, the target rock lithology (i.e. Earth's earliest crust) would be between the composition of quartz-rich granitoid and tonalite-trondhjemite-granodiorite.