The quest for earth's oldest crust

Geologists have been probing various continental terrains in search of possible remnants of earliest crust that may have survived more than four billion years (b.y.) of earth’s history, the first 10 to 100 million years (m.y.) of which were catastrophic, marked by intense volcanism and impacts from meteorites. Earlier views of a globe circling veneer of ‘sialic’ crust arising from a residuum of crystallizing molten earth, are not favoured and it is now believed that the pre-4 b.y. crusts were essentially oceanic basalts or possibly high magnesian komatiites produced under the prevailing high temperature conditions. In fact, crust formation studies, based on Nd isotopes, have revealed that roughly 8% of crust had formed by the time the earth was 0.5 b.y. old and about 60%, by 2.6 b.y. (refs 1, 4). Now, the discovery of 4.14–4.28 b.y. old detrital zircons within younger sedimentary strata in Mt. Narryer complex in western Australia and a few near 4 b.y. old zircons from other places have spurred interest among the geological community to look for their provenance or parent rocks. Since zircons occur as accessories, mostly in granites, it is likely that their source rocks were granitic, which must have remained stable for periods long enough to be weathered down to provide materials, including these zircons for the younger sedimentary sequences. For the past two decades, attempts by geologists to find crusts that had formed between 4.0 and 4.5 b.y. could lead them only close to the 4 b.y. mark. The Acasta gneissic area in the Slave Province (NW Territories, Canada) forming part of a mesoto palaeo-Archaean continental crust in the Canadian Shield was found retaining rocks older than 3.6 b.y., the oldest among them being 3.96 b.y. (ref. 8). Exposures quite close in age to these gneisses occur in western Greenland (3.8–3.6 b.y. Itsaq gneisses), MontanaWyoming in USA (3.4 b.y. gneisses), W. Australia (3.63 b.y. orthogneiss), Antarctica (3.9 b.y. granulites, Enderby Land) and China (3.8 b.y. Sino-Korean Craton) (Figure 1). In India, 3.0 to 3.3 b.y. rocks have been reported in Singbhum, N. Orissa (tonalite gneisses within Singhbhum granite), Karnataka (Gorur-Hassan gneiss), Goa (trondjemite gneiss) and Rajasthan (amphibolite, Udaipur). The discovery of such crusts in India and elsewhere, formed close to 4 b.y. mark (although the slightly higher ages reported are contested) indicating development of stable geological conditions by then, had raised hopes for locating still older relicts. In this quest over the last few years, field and laboratory investigations by geologists from Canada, USA and Australia in the Acasta gneissic region pushed the record of rock age closer than ever before to the 4.5 b.y. mark. Recognition of rocks formed within 0.5 b.y. of earth formation requires a precise isotope dating technique and availability of suitable materials preserving in toto the radiogenic products to testify their antiquity. The mineral zircon, highly resistant to weathering processes and low-grade metamorphism, is ideal in this respect. However, alpha particles from decaying uranium, present in small amounts, damage the mineral’s lattice (metamictization) making it susceptible to alteration and consequent leak

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