RAPID CHANGES IN CRUSTAL DIFFERENTIATION ACROSS THE ARCHEAN-PROTEROZOIC BOUNDARY

The process of crustal differentiation, which converts magmas of broadly basaltic composition to more silicic compositions that typify continental crust, is required to sustain Earth’s bimodal hypsometry and an efficient silicate weathering feedback on Gyr timescales. Though some quantity of felsic crust has likely been present on Earth since the Hadean, the process and mechanism of crustal differentiation is only partially understood for the modern Earth, and very poorly constrained in the early Earth. We extend the approach of Keller and Schoene (2012) to quantify secular temporal variation in the enrichment and depletion of major and trace elements during the process of crustal differentiation from the early Archean to the present. While we find no evidence for a change in the occurrence or prevalence of subduction-driven flux melting (and thus plate tectonics) as a source for continental basaltic magmatism, the geochemistry of crustal differentiation does change dramatically near the Archean-Proterozoic boundary. The results require greater garnet fractionation and diminished plagioclase fractionation in the Archean, a signature which appears inescapably tied to greater pressures of magmatic differentiation in the Archean. However, the geologic causes for this shift remain elusive. A full understanding of the preserved continental record of crustal differentiation will likely require greater integration of the geochemical record with geophysical variables such as preserved crustal thickness, crustal velocity structure, and crustal density — as well as further integration with our record of surface earth processes such as atmospheric oxygenation across the Archean-Proterozoic boundary.