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

2012; Nature Portfolio; Volume: 485; Issue: 7399 Linguagem: Inglês

10.1038/nature11024

1476-4687

C. Brenhin Keller, Blair Schoene,

earthquake and tectonic studies

Statistical sampling of a large geochemical database reveals a pervasive discontinuity about 2.5 billion years ago, indicating marked changes in mantle and deep-crustal melting, and providing a link between deep Earth processes and the rise of atmospheric oxygen on the Earth. Brenhin Keller and Blaire Schoene apply statistical sampling techniques to a geochemical database of about 70,000 samples from continental igneous rocks to produce a record of secular geochemical evolution throughout Earth's history. They find, superimposed on the expected gradual geochemical evolution attributable to secular cooling of Earth, a pervasive geochemical discontinuity approximately 2.5 billion years ago. This discontinuity is indicative of dramatic decreases in mantle melt fraction in basalts and in deep crustal melting/fractionation indicators. The Archaean/Proterozoic geochemical transition revealed by this analysis coincides with sudden atmospheric oxygenation at the end of the Archaean aeon, providing a temporal link between deep Earth geochemistry and the rise of atmospheric oxygen. The Earth has cooled over the past 4.5 billion years (Gyr) as a result of surface heat loss and declining radiogenic heat production. Igneous geochemistry has been used to understand how changing heat flux influenced Archaean geodynamics1,2, but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias3,4,5. Here we apply statistical sampling techniques to a geochemical database of about 70,000 samples from the continental igneous rock record to produce a 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 occurring about 2.5 Gyr ago, involving substantial decreases in mantle melt fraction in basalts, and in indicators of deep crustal melting and fractionation, such as Na/K, Eu/Eu* (europium anomaly4) and La/Yb ratios in felsic rocks. Along with an increase in preserved crustal thickness across the Archaean/Proterozoic boundary6,7, these data are consistent with a model in which high-degree Archaean mantle melting produced a thick, mafic lower crust and consequent deep crustal delamination and melting—leading to abundant tonalite–trondhjemite–granodiorite magmatism and a thin preserved Archaean crust. The coincidence of the observed changes in geochemistry and crustal thickness with stepwise atmospheric oxidation8 at the end of the Archaean eon provides a significant temporal link between deep Earth geochemical processes and the rise of atmospheric oxygen on the Earth.