Statistical geochemistry reveals disruption in secular lithospheric evolution about 2.5 Gyr ago

Secular cooling of the Earth is required by surface heat loss and declining radiogenic heat production over the last 4.5 billion years. Igneous geochemistry has been used to understand how changing heat flux influenced Archean geodynamics, but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias. We apply statistical sampling techniques to a geochemical database of ~70,000 samples from the continental igneous rock record to produce the first comprehensive record of secular geochemical evolution throughout Earth history. Consistent with secular mantle cooling, compatible and incompatible elements in basalts record gradually decreasing mantle melt fraction through time. Superimposed on this gradual evolution is a pervasive geochemical discontinuity ca. 2.5 Ga, including dramatic decreases in mantle melt fraction in basalts, and in deep crustal melting/fractionation indicators such as Na/K, Eu/Eu*, and La/Yb of felsic rocks. Along with increased preserved crustal thickness across the Archean-Proterozoic boundary, these data are consistent with a model where high degree Archean mantle melting produced a thick, mafic lower crust and consequent deep crustal delamination and anatexis – in turn resulting in abundant tonalite-tronhjemite-granodiorite (TTG) magmatism and a thin preserved Archean crust. Coincidence of the observed changes in geochemistry and crustal thickness with stepwise atmospheric oxidation at the end of the Archean provides the most significant temporal link between deep earth geochemical processes and the rise of atmospheric oxygen on Earth.