No significant production of continental crust prior to 3.8 Ga

In his final paper before his death in 1991 (The Persistent Myth of Crustal Growth), Dick Armstrong argued that continental crustal recycling has kept pace with crustal growth and that the volume of continental crust has been in an approximate steady state (no net growth) for at least the past 4 Ga. Although this has been a minority viewpoint (most scientists favor some manner of progressive crustal growth) Armstrong's no-growth model has had some vehement supporters over the years. Since this paper was published, most of the discussion in the literature has focused on the isotopic record—particularly that of the oldest preserved igneous rocks. Recently, attention has shifted to detrital zircons and, in particular, the Hf isotopic record in the oldest known zircons. Two main features from these data have been used to argue for widespread crust in the early Earth: 1) heterogeneous Hf isotopic compositions of the oldest zircons; and 2) Hf model ages in Archean detrital zircons that are much older than their crystallization ages. The dataset on which these observations have been based, however, is not robust. Many data are compromised by complexities in age and isotopic composition. Hf zircon model ages are even more problematic as they are loaded with assumptions and based on a depleted mantle evolution reference that is not well known, is most likely wrong prior to 3.8 Ga, and has uncertain relevance for understanding crustal growth. In order to provide a less ambiguous isotopic record, a better approach is needed: the integration of age (U-Pb zircon) and isotopic information (Hf-Nd whole rock, Hf-O in zircons) from the oldest, but also least complicated and altered, magmatic samples. Several lines of evidence suggest that formation of continental crust did not begin in earnest until ca. 3.8-3.5 Ga: 1) lack of older crust or inherited pre-4 Ga zircon cores in the geological record in shield areas; 2) conspicuous lack of pre-4 Ga zircons in nearly all Archean sedimentary rocks (if zircon bearing continental crust existed in present-day volumes prior to 4.0 Ga, it was destroyed virtually without a trace in even the oldest sediments); 3) Pb and other isotopic data from Archean terranes show evidence of older Archean crust only where it is known to exist today (i.e., Slave and Pilbara cratons) but not elsewhere; 4) The best constrained Hf isotope data of mantle-derived samples show that the depleted mantle does not diverge significantly from bulk Earth (based on CHUR) until ca. 3.8-3.5 Ga, or even later if the Earth has a non-chondritic composition. We agree with Armstrong that it is "inescapable" that the Earth differentiated into depleted and enriched reservoirs soon after planetary accretion, but disagree that the enriched reservoir was necessarily an evolved crust typical of modern continental crust; it is more likely to be mafic and devoid of zircon. An essential part of crustal growth, therefore, is not only in the extraction of enriched material from the mantle but also in the transformation of this dominantly basaltic protocrust into something progressively more felsic, with greater preservation potential. This process begins to pick up steam at ca. 3.8 Ga.