The early formation of the continental crust: constraints from zircon Hf-isotope data

A worldwide database of >16,000 U-Pb and Hf-isotope analyses of zircon, largely from detrital sources, has been interogated to analyse the processes of crustal evolution on a global scale, and to test existing models for the growth of continental crust through time. At any timeslice, most zircons have ε_Hf well below the Depleted Mantle (DM) growth curve, reflecting later reworking of originally juvenile material. To quantitatively estimate the proportion of juvenile material added to the crust at any given time during its evolution, it is necessary to correct for this effect. "Crustal" model ages are calculated assuming the zircon-bearing magmas were generated from the average continental crust ¹⁷⁶Lu/¹⁷⁷Hf = 0.015); zircons with non-juvenile Hf are projected back to the DM growth curve using this ratio. Juvenile magmas are defined as having ε_Hf > or =0.75 times the epsilon (sub Hf) of the DM at the time of genesis. The distribution of corrected model ages can then be used to model the true crustal growth rate over the 4.56 Ga of Earth's history. The modelling shows that there was little episodicity in the production of new crust, as opposed to peaks in magmatic ages. The distribution of age-Hf isotope data from zircons worldwide implies that at least 60% of the existing continental crust separated from the mantle before 2.5 Ga, and has been variably reworked since then. However, taking into consideration new evidence coming from geophysical data, and correcting for the geographical biases in database, the formation of most continental crust still earlier in Earth's history (at least 70% before 2.5 Ga) is even more probable. Thus, crustal reworking has dominated over net juvenile additions to the continental crust, at least since the end of the Archean. Moreover, the juvenile proportion of newly formed crust in any timeslice decreases stepwise through time: it is about 70% in the 4.0-2.2 Ga time interval, about 50% in the 1.8-0.6 Ga time interval, and possibly less than 50% after 0.6 Ga. These changes may be related to the formation of supercontinents.