Geocheminar winter-2014

The evolution of the Earth's earliest crust remains largely unknown due to the dearth of Hadean (>4 Ga) rocks, with most observational evidence of the planet's first few hundred million years deriving from geochemical studies of 4.4-4.0 Ga detrital zircons from Jack Hills, Narryer Gneiss Complex (Yilgarn craton). Previous Lu-Hf investigations of these zircons suggested that continental-like (low Lu/Hf) crust formation began by ~4.4-4.5 Ga and continued for several hundred million years. The most isotopically primitive crust represented in the Jack Hills population was preserved until at least ~4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even in the Jack Hills zircon population, the most unradiogenic, ancient isotopic signals have not been identified in the younger (<3.9 Ga) rock and zircon record. We present new Lu-Hf results from <4 Ga Jack Hills zircons that indicate a significant transition in Yilgarn crustal evolution between 3.9 and 3.7 Ga. The Jack Hills zircon protolith evolves largely by internal reworking through the period 4.0 to 3.8 Ga, and both the most ancient and unradiogenic components of the crust are 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, a combination of prior oxygen isotope measurements along with new trace element measurements shows that this period is also characterized by a restriction in d18O and an overall shift in several zircon trace element characteristics after ca. 3.9-3.8 Ga. The simultaneous loss of ancient crust accompanied by juvenile crust addition is best explained by a mechanism similar to subduction, by which both processes are effected on the modern Earth. The other geochemical information also supports a transition in zircon formation environment in this period, although it is less sensitive to processes like crustal recycling. We interpret these data as consistent with the action of destructive plate boundaries by the Eoarchean. Additionally, the crustal loss event identified here may be useful for future comparisons of the fate of Hadean crust in other Archean terranes.