Barium content of Archaean continental crust reveals the onset of subduction was not global

Earth's earliest continental crust is dominated by tonalite-trondhjemite-granodiorite (TTG) suites, making these rocks key to unlocking the global geodynamic regime operating during the Archaean (4.0-2.5 billion years ago [Ga]). The tectonic setting of TTG magmatism is controversial, with hypotheses arguing both for and against subduction. Here we conduct petrological modeling over a range of pressure-temperature conditions relevant to the Archaean geothermal gradient. Using an average enriched Archaean basaltic source composition, we predict Ba concentrations in TTG suites, which is difficult to increase after magma generated in the source. The results indicate only low geothermal gradients corresponding to hot subduction zones produce Ba-rich TTG, thus Ba represents a proxy for the onset of subduction. We then identify statistically significant increases in the Ba contents of TTG suites worldwide as recording the diachronous onset of subduction from regional at 4 Ga to globally complete sometime after 2.7 Ga. Only subduction zone can produce Ba-rich TTG, representing a proxy for the onset of subduction. Statistical increases in Ba contents of Archaean TTGs reveal the diachronous onset of subduction from regional at 4 Ga to globally complete after 2.7 Ga