Considerable geochemical evidence supports initiation of plate tectonics on Earth shortly after the end of the Hadean. Nb/Th and Th/U of mafic-ultramafic rocks from the depleted upper mantle began to change from 7 to 18.2 and 4.2 to 2.6 (respectively) at 3.6 Ga. This signals the appearance of subduction-altered slabs in general mantle circulation from subduction initiated by 3.9 Ga. Juvenile crustal rocks began to show derivation from progressively depleted mantle with typical igneous ɛ Nd : ɛ Hf = 1:2 after 3.6 Ga. Cratons with stable mantle keels that have subduction imprints began to appear by at least 3.5 Ga. These changes all suggest that extraction of continental crust by plate tectonic processes was progressively depleting the mantle from 3.6 Ga onwards. Neoarchean subduction appears largely analogous to present subduction except in being able to produce large cratons with thick mantle keels. The earliest Eoarchean juvenile rocks and Hadean zircons have isotopic compositions that reflect the integrated effects of separation of an early enriched reservoir and fractionation of Ca-silicate and Mg-silicate perovskite from the terrestrial magma oceans associated with Earth accretion and Moon formation, superposed on subsequent crustal processes. Hadean zircons most likely were derived from a continent-absent, mafic to ultramafic protocrust that was multiply remelted between 4.4 and 4.0 Ga under wet conditions to produce evolved felsic rocks. If the protocrust was produced by global mantle overturn at ca. 4.4 Ga, then the transition to plate tectonics resulted from radioactive decay-driven mantle heating. Alternatively, if the protocrust was produced by typical mantle convection, then the transition to plate tectonics resulted from cooling to the extent that large lithospheric plates stabilized.

Abstract Ages in the range 3.6–4.0 Ga (billion years) have been reported for the oldest, continental, granitoid orthogneisses, whose magmatic precursors were probably formed by partial melting or differentiation from a mafic, mantle-derived source. The geological interpretation of some of the oldest ages in this range is still strongly disputed. The oldest known supracrustal (i.e. volcanic and sedimentary) rocks, with an age of 3.7–3.8 Ga, occur in West Greenland. They were deposited in water, and several of the sediments contain 13 C-depleted graphite microparticles, which have been claimed to be biogenic. Ancient sediments ( c . 3 Ga) in western Australia contain much older detrital zircons with dates ranging up to 4.4 Ga. The nature and origin of their source is highly debatable. Some ancient (magmatic) orthogneisses ( c . 3.65–3.75 Ga) contain inherited zircons with dates up to c . 4.0 Ga. To clarify whether zircons in orthogneisses are inherited from an older source region or cogenetic with their host rock, it is desirable to combine imaging studies and U-Pb dating of single zircon grains with independent dating of the host rock by other methods, including Sm-Nd, Lu-Hf and Pb/Pb. Initial Nd, Hf and Pb isotopic ratios of ancient orthogneisses are essential parameters for investigating the degree of heterogeneity of early Archaean mantle. The simplest interpretation of existing isotopic data is for a slightly depleted, close-to-chondritic, essentially homogeneous early Archaean mantle; this does not favour the existence of a sizeable, permanent continental crust in the early Archaean. By analogy with the moon, massive bolide impacts probably terminated on Earth by c . 3.8–3.9 Ga, although no evidence for them has yet been found. By c . 3.65 Ga production of continental crust was well underway, and global tectonic and petrogenetic regimes increasingly resembled those of later epochs.