|Title:||Eoarchean crustal evolution of the Jack Hills zircon source and loss of Hadean crust|
|Authors:||E. A. Bell, T. M. Harrison, I. E. Kohl, and E. D. Young|
|Publication:||Geochim. Cosmochim. Acta., v. 146, p. 27‐42.|
Given the global dearth of Hadean (> Ga) rocks, 4.4–4.0 Ga detrital zircons from Jack Hills, Narryer Gneiss Complex (Yilgarn Craton, Western Australia) constitute our best archive of early terrestrial materials. Previous Lu–Hf investigations of these zircons suggested that felsic (low Lu/Hf) crust formation began by ⁓ 4.4 to 4.5 Ga and continued for several hundred million years with evidence of the least radiogenic Hf component persisting until at least ⁓ 4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even in the detrital Jack Hills zircon population, the most unradiogenic, ancient isotopic signals have not been definitively identified in the younger (<.9 Ga) rock and zircon record. Here we show Lu–Hf data from < Ga Jack Hills detrital zircons that document a significant and previously unknown transition in Yilgarn Craton crustal evolution between 3.9 and 3.7 Ga. The zircon source region evolved largely by internal reworking through the period 4.0–3.8 Ga, and the most ancient and unradiogenic components of the crust are mostly missing from the record after ⁓ 4 Ga. New juvenile additions to the crust at ca. 3.9–3.8 Ga are accompanied by the disappearance of unradiogenic crust ca. 3.9–3.7 Ga. Additionally, this period is also characterized by a restricted range of δ18O after 3.8 Ga and a shift in several zircon trace element characteristics ca. 3.9–3.6 Ga. The simultaneous loss of ancient crust accompanied by juvenile crust addition can be explained by a mechanism similar to subduction, which effects both processes on modern Earth. The oxygen isotope and trace element information, although less sensitive to tectonic setting, also supports a transition in zircon formation environment in this period.