Australia: The Land Where Time Began

A biography of the Australian continent 

Jack Hills Zircons – Hadean Crustal Evolution Revisited, New Constraints From Pb-Hf Systematics

The only surviving vestiges of crust from the Hadean are zircon crystals that are found in the Jack Hills in Western Australia, which Kemp et al. say represent an extraordinary archive into the nature of the early Earth. In this paper they report the results of an in situ study of 68 zircons from Jack Hills in which they measured concurrently the isotope ratios of Hf and Pb, which allowed a better integration of tracer information (¹⁷⁶Hf/¹⁷⁷Hf) with age of crystallisation (²⁰⁷Pb/²⁰⁶Pb). These data were augmented by Hf isotope data from zircons recovered from the surrounding Narryer gneisses of 3.65-3.30 Ga which places the time period of the granites that intrude the Jack Hills belt as Neoarchaean. A subchondritic ε_Hf –time array that attests to an evolution of the Hadean Earth that is far simpler than claimed by recent studies. According to Kemp et al. this evolution is consistent with the protracted intra-crustal reworking of an enriched protolith, that was dominantly mafic, that had been extracted from primordial mantle 4.4-4.5 Ga, possibly during the solidification of a terrestrial ocean of magma. Kemp et al. say there is no evidence of the existence of Hadean mantle that has been strongly depleted, or of juvenile input into parental magmas to the Jack Hills zircons. It is difficult to reconcile this simple Hf isotope evolution with the processes of modern plate tectonics. It is suggested by strongly unradiogenic Hf isotope compositions of zircons that have been recovered from gneiss terranes of Archaean age, including the Narryer and Acasta (Canada) gneisses, that the Hadean source reservoirs were tapped by granitic magmas throughout the Archaean. This supports the notion of a protocrust in the Hadean that was long-lived and globally-extensive

Sources & Further reading

Kemp, A.I.S., et al.,Hadean crustal evolution revisited: New constraints from Pb–Hf isotope systematics of the Jack Hills zircons, Earth Planet. Sci. Lett. (2010), doi:10.1016/j.epsl.2010.04.043