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

Abstract

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.

Fig. 1. Compositions of tonalite–trondhjemite–granodiorite (TTG) suites.

a Mg# versus MgO. b–d K₂O/Na₂O, Sr/Y, and Ni contents versus Ba contents. Data are from ref. .

Fig. 2. Phase diagrams for an average Archaean tholeiitic basalt.

a A simplified pressure–temperature (P–T) diagram of an average Archaean tholeiitic basalt composition, labeled with the mol.% of melts. b, c mineral and melts proportions versus temperature, along 450 °C GPa⁻¹ and 900 °C GPa⁻¹, respectively.

Fig. 3. Modeled tonalite–trondhjemite–granodiorite (TTG) melts compositions.

a Mg# versus MgO content of modeled melts, experiment data from similar protolith composition was also plot for comparison. Numbers in the legend show the thermal gradient. XP indicates experimental results. b–d K₂O/Na₂O, Sr/Y, and Ni contents versus Ba contents, concordant with natural samples in Fig. 1. The circles represent the calculated melt fractions at 2 mol.% intervals. The background color change from yellow to orange (cold to hot, respectively) indicates the direction of temperature increase.

Fig. 4. Ba content of Archaean tonalite–trondhjemite–granodiorite (TTG) suites.

a Ba contents of TTG suites through the Archaean. The gray and black solid circles represent the real and binned compositions of natural TTG suites, respectively. Data are from ref. . b Correlation between age of Ba step change with the oldest rock by craton (degree 2 polynomial regression).

Fig. 5. Modeled thermal histories.

Modeled thermal histories with a switch in heat-flow scaling from stagnant-lid convection to plate tectonics at 1 Ga (blue), 2 Ga (yellow), 3 Ga (orange), and 4 Ga (red), while a case entirely with plate tectonics scaling is shown in solid (see ref. for modeling details). Red box shows onset range for global subduction with 2.7 Ga as an upper bound (this study) and 2.0 Ga as a lower bound, consistent with estimates of mantle potential temperature (T_p) as compared to thermal modeling.