Statistical analysis on secular records of igneous geochemistry: Implication for the early Archean plate tectonics

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.