Diverse genesis of early Earth’s continental crust hints the geodynamic transition at about 3.0 Gyrs ago

As the dominant component of Earth's early continental crust, tonalite-trondhjemite-granodiorite (TTG) suites offer critical insights into the crust-mantle dynamic systems and geodynamic regime for the early Earth (>2.5 Ga). Although TTGs are generally accepted to have originated from partial melting of hydrated metabasalt, specific conditions and mechanisms remain enigmatic, which has sparked intense debate over the geodynamic settings of the early Earth. Here, we conduct thermodynamic-geochemical modellings to systematically compare the roles that pressure, bulk H₂O content, and source rock composition play in shaping TTG magmas. We find that pressure is the first-order factor controlling the formation and compositional diversity of TTG. Our modellings also predict the optimal melting conditions for different types of TTGs, which are further validated by the ranges of magmatic H₂O contents recorded by apatite and zircon from global TTG samples. We propose an apatite-based melt hygrometer and apply it to Archean TTGs for the first time. Combined with the results from the zircon hygrometer, our data show that high-pressure TTGs have the highest H₂O contents (7 – 12 wt.%), whereas the low-pressure TTGs have the lowest (4 – 7 wt.%), matching our prediction of the optimal H₂O contents for TTG melts. We show that high-pressure TTG is likely derived from fluid-fluxed melting at subcrustal depths (14 – 16 kbar), a process readily explained by subduction rather than intraplate crustal formation models. Furthermore, the temporal and spatial distribution of both high-pressure TTGs and arc-like basalts points to subduction that likely started as a localized phenomenon and transitioned to a global-scale process at about 3.0 Ga.