Post-Hadean transitions in Jack Hills zircon provenance: A signal of the Late Heavy Bombardment?

Abstract

Hadean Jack Hills (Western Australia) detrital zircons represent the best documented terrestrial resource with which to observe the pre-4Ga Earth. The >4Ga component of this semi-continuous 4.38 to 3.0Ga zircon record has been investigated in detail for age, δ18O, Lu–Hf systematics, and Ti thermometry. The more abundant post-Hadean population is less well-characterized, but a previous study (Bell et al., 2011) suggests a more restricted range of δ18O source materials together with a ca. 4.0–3.6Ga discontinuity in Lu–Hf evolution. These differences could reflect a transformation in the character of the older zircon source region or their sourcing from different terranes entirely. The relative scarcity of 4.0–3.6Ga zircons corresponds to a discontinuity in Lu–Hf evolution after which 176Hf/177Hf in zircon reverts to more radiogenic values relative to the >4Ga population. We present new oxygen isotope, titanium, and trace element results for 4.0–3.6Ga Jack Hills zircons in a search for apparent transitions in petrological conditions. Post-3.8Ga zircons show a marked decrease in the occurrence of heavy oxygen (>6.5‰), but remain close to the average of the Hadean distribution despite their restricted range. This may point to the decreased importance of sedimentary materials in post-3.8Ga magmas. Ca. 3.9Ga zircons fall into two categories: “Group I” displays temperatures and compositions similar to the Hadean zircons whereas “Group II” zircons have higher U and Hf, and lower (Th/U), Ce and P. Group II zircons also have anomalously low Ti, and are remarkably concordant in the U–Pb system. Group II's geochemical characteristics are consistent with formation by transgressive recrystallization (Hoskin and Black, 2000), in which non-essential structural constituents are purged during high-grade thermal metamorphism. The restricted age range of Group II occurrence (3.91–3.84) and its coincidence with the postulated intense bolide flux in the inner solar system (i.e., Late Heavy Bombardment; 3.95–3.85) may have causal significance.