Statistical Geochemistry Reveals Geochemical Evolution of the Continental Crust

Brenhin Keller, Princeton University

Photo of Brenhin Keller

The composition of Earth’s crust does not remain constant through geologic history, but rather changes as a function of mantle heat flux and crustal tectonics. However, the extreme heterogeneity of the preserved rock record has obstructed the quantification of crustal composition and of its evolution over time. The increasing availability of large compiled geochemical datasets and suitable computational tools to analyze them provides a new opportunity to see through the heterogeneity of the rock record and identify compositional trends through time. We have conducted weighted bootstrap resampling and Monte Carlo analysis on a database of more than 500,000 igneous rock analyses to produce the first quantitative record of crustal geochemical evolution. In addition to gradual trends in compatible and incompatible element abundance expected from mantle cooling, the results reveal a period of dramatic geochemical change near the Archean/Proterozoic boundary. Temporal correlation of this event with stepwise atmospheric oxidation circa 2.4 Ga may suggest a link between deep-earth geochemical processes and the rise of atmospheric oxygen on Earth, a conjecture supported by expected changes in iron redox partitioning with crustal melting pressure. Additional analysis of the dataset suggests that the major geochemical discontinuity ca. 2.5 Ga is associated with changing systematics of mantle melting, with consequences for the expected style of crust formation and plate tectonics through time. Further, application of high-performance computing to the dataset has allowed us to conduct thermodynamic simulations on each analyzed rock sample, producing better estimates of the P-T conditions of mantle melting and of the nature and extent of subsequent magmatic fractionation. Together, these constraints allow us to work backwards in time, in a justified application of uniformitarianism, to infer the style of Archean tectonics and the initiation of oceanic subduction.

Abstract Author(s): C. Brenhin Keller, Blair Schoene