Abstract
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 geodynamics, but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias. 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 anomaly) and La/Yb ratios in felsic rocks. Along with an increase in preserved crustal thickness across the Archaean/Proterozoic boundary, 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 oxidation 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.
Highlights
Secular cooling of the Earth is required by surface heat loss and declining radiogenic heat production over the last 4.5 billion years
Resampling weights are inversely related to the high-Na TTG series, and the purported structural evolution spatiotemporal sample density in order to achieve a maximally of granite-greenstone terranes have all been used to suggest uniform posterior sample density distribution and minimize sethat plate tectonics may have been different in style or even lection bias
The reported occurrence of subduction signatures to distinguish between primary mantle melting and subsequent such as high-field-strength element depletion have led other au- evolution and fractionation processes, we extract results from thors to argue that plate tectonics has existed in its present state two silica ranges: 62-74% SiO2 for intermediate to felsic samsince at least 3.0 or as early as 3.8 Ga [9, 10]
Summary
A database of major and trace element analyses of ∼70,000 igneous whole-rock samples was assembled from a number of preexisting sources, including ∼2500 from Condie [11], Condie and O’Neill [3], ∼1500 from Moyen [4], Moyen and Stevens [20], and the remaining majority from the EarthChem repository (encompassing samples from Navdat, Georoc, and the USGS) [13]. Geochemical data for individual samples was coupled with crustal and lithospheric thickness, crustal Vp, Vs, and density by superimposing sample locations onto high spatial-resolution global crustal and lithospheric models CRUST 2.0 [14] and TC1 [5], respectively. The result was a dataset including up to 98 defined variables (e.g., sample age, SiO2, La, crustal thickness, etc.) for each of the ∼70,000 samples, for a total of more than 3.9 million defined datapoints
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