The creation and preservation of Archaean continental crust

Thirty years ago Armstrong (1968) proposed that the crust of the Earth was formed by 4.0 Ga and since this time steady-state recycling has maintained it at a constant volume at the surface of the Earth. In his model, the small proportion of old rocks preserved at the surface today were simply remnants of crust that survived the recycling process. More recent models of crustal growth (i.e. All~gre and Rousseau, 1984; McCulloch and Bennett, 1994) invoke the gradual growth of continental crust, punctuated by a series of peaks in production. The latter models imply a relatively simple process of progressive depletion of the upper mantle and growth of crust, in which, by the end of the Al'chaean (2,5 Ga), only 50-60% of the crust had been formed. The input parameters to the models are the radiogenic isotopes of Sr, Nd and Pb, and, to a lesser extent constraints from U/Pb zircon distribution. The Pb and U balance of crust, OIB and MORB provide important constraints on the amount of sediment that is recycled into the mantle, and, based on current fluxes, most models cannot accommodate >10% of recycled sediment. In recent years improvements in analytical techniques have made it possible to obtain reliable estimates of key trace element ratios in Archaean basaltic and ultramafic material (i.e. Jochum et al., 1991) and these authors argue that the sub-chondritic ratios of Nb/U and the depletion in epsilon-Nd in the early Earth are consistent with extraction of a proto-crust, since destroyed. In recent years several authors have stressed the role of accreted large igneous provinces, rather than arc accretion in crustal growth (Kusky and Kidd, 1992, Abouchami et al., 1990). These arguments were in large part based on the identification of the giant mafic-ultramafic basaltic provinces in the ocean basins (Coffin and Eldholm, 1994), and the detection of large tracts of geochemically juvenile material with oceanic plateau characteristics in ancient greenstone sequences.