Geochemical Constraints on the Growth of the Continental Crust

Several lines of evidence indicate that the Archean upper crust was considerably more mafic than the present-day upper crust. There has been no significant change in REE and Th abundances in post-Archean clastic sedimentary rocks, suggesting that there has been no change in the composition of the upper crust during the post-Archean. This indicates that if there have been any additions to the post-Archean upper crust, they must have had similar composition to the upper crust itself. Geochemical modelling of REE and Th abundances in sedimentary rocks suggests that the minimum ratio of post-Archean to Archean upper crustal composition required to eliminate the Archean upper crustal trace element signature, within analytical uncertainty, is about 4:1. Such a model also is supported by isotopic data. Using plausible assumptions regarding the volume of Archean crust, isostatic relations, and extreme models of the earth's degassing history, it is proposed that approximately 65-75% of the continental crust formed during the period of 3.2-2.5 Ga, and that between 70-85% of the continental crust had formed by 2.5 Ga. Such a model is consistent with relatively constant continental freeboard during the past 2,500 million years. Thus, the constant freeboard model does not provide unique evidence for large-scale recycling of continental material through the mantle.