In recent years, application of statistical analysis to a sufficiently large geochemical database has become more and more popular to reveal the onset and evolution of plate tectonics as a consequence of the development of computer science and storage technology. Here, we introduce a robust statistical method to process a filtered geochemical dataset including predominantly continental basaltic rocks through Earth's history. The results show that the average Sr concentrations gradually increased from 3.8 to 0 Ga, indicating a progressively growing depth of magma source as a result of secular cooling of the mantle. However, the average La/Yb and Sm/Yb ratios only started to increase at ca . 3.0 Ga. Considering the La–Yb and Sm–Yb fractionations are commonly attributed to the residual garnet crystallization, we interpreted that the geotherm of the magma source of continental basaltic rocks before 3.0 Ga might be higher than the stability limit of garnet. As such, there were little or no garnets in the residue during partial melting of the upwelling asthenospheric mantle or mantle plumes before 3.0 Ga, which also implied a higher continental geotherm during that time. Such a high geotherm could not support the formation of contemporaneous high‐pressure or medium‐pressure TTGs (tonalite–trondhjemite–granodiorite) by vertical tectonic models associated with upwelling mantle heating or delaminated lower crust melting in the early Archean. We therefore conclude that the plate tectonics had occurred by ca . 3.8 Ga in a hot subduction style and contributed to the formations of high‐ and medium‐pressure TTGs subsequently.
Mobilities and fractionations of high-field-strength elements, especially Nb and Ta within a subducting slab, are important for deciphering the formation of the continental crust (CC). Here we report geochemical results on an epidote garnet amphibolite facies metagabbro body in the Tongbai-Dabie orogenic belt, central China. Our samples were hydrated during prograde metamorphism of the Triassic plate subduction. Major minerals such as amphibole, garnet, rutile, and ilmenite and garnet amphibolite bulk rocks show varied and overall lower Nb/Ta and/or Zr/Hf ratios than the continental crust. Magma differentiation might have contributed to variations of Zr/Hf but not those of Nb/Ta, suggesting major Nb/Ta fractionations during plate subduction. LA-ICPMS in situ trace element analyses of amphibole and especially rutile grains exhibit obvious chemical zonations. Typically, the rutile cores are usually small with higher Nb and Ta concentrations and lower Nb/Ta ratios compared to the thick rims. Chemical and fabric characteristics of the zonations may be explained by diverse external fluid activities: the gabbro first absorbed low Nb/Ta fluids that were released during blueschist to amphibolite transformation in deeper portions of the subducting slab, followed by acquiring external fluids with elevated Nb/Ta released during amphibolite to eclogite transformation. Our results imply that fluids with low Nb/Ta released during blueschist to amphibolite transformation can be transferred to cold regions within a subducted plate and also to the mantle wedge through fluid-rock reaction. Such regions are more easily melted during further subduction, especially in the early history of the earth, providing a plausible explanation for the low Nb/Ta in the CC.
Abstract Over the Earth’s evolutionary history, the style of plate subduction has evolved through time due to the secular cooling of the mantle. While continuous subduction is a typical feature of modern plate tectonics, a stagnant-lid tectonic regime with localized episodic subduction likely characterized the early Earth. The timing of the transition between these two subduction styles bears important insights into Earth’s cooling history. Here we apply a statistical analysis to a large geochemical dataset of mafic rocks spanning the last 3.5 Ga, which shows an increasing magnitude of alkali basaltic magmatism beginning at ca. 2.1 Ga. We propose that the rapid rise of continental alkali basalts correlates with an abruptly decreasing degree of mantle melting resulting from the enhanced cooling of the mantle at ca. 2.1 Ga. This might be a consequence of the initiation of continuous subduction, which recycled increasing volumes of cold oceanic crust into the mantle.
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