Not sure about the origin of first continent, however probably not the subduction

The formation mechanism of first stable continental crust has been discussed over decade. One of hypothesis is slab melting, which can explain trace element patterns of the Archean TTGs. Here, Johnson et al.[1] erase one distractor, the subduction.

They used The East Pilbara Terrane for their study. They assumed the Paleoarchean TTG as the first stable continent, and the Coucal basalt as their parent rock because, the TTG needs a source that are enriched in large ion lithophile element than MORB.

For suitable major and trace element composition of TTG, the Coucal basalt should had melt for 20~30% in geotherm gradient of 900˚C , which is a stable condition for garnet (which takes HREE), but not for rutile.

Just like TTG, the Coucal basalt’s major and trace element compositions show that the Coucal basalt’s parent rock is not mantle. The Coucal basalt has another step of parent rock, which has high Mg#. As we can see in Figure 1, the major element composition of TTG from 20~30% melted the Coucal basalt shows high similarity with Coucal basalt, likewise the trace element composition. This means the “arc-like” composition of TTGs does not directly point out arc environment.

To explain the Coucal basalt and TTG’s compositions, the lower crust environment’s geotherm gradient and temperature condition has suggested. Under the crust, the drip tectonic can provide multiple melting steps to underlying materials. And these steps seem necessary to explain 300million years of TTG formation process from mantle melt, recorded by Sm/Nd isotope system, and the Coucal basalt’s non-primary mantle melt composition.

In summary, the TTG’s parent rock was not the MORB, and its “arc-like” characteristics don’t directly mean arc environment. Rather TTG’s formation underwent multiple melting steps, comprise enriched basalts like the Coucal basalt.

Figure 1 Calculated composition of Major elements of TTGs at given rate of melting. Geotherm gradient has calculated as 900˚C . Outline of grey area indicates 2σ envelope, which means over 95.45% of similarity.

[1] Johnson, Tim et al.(2017), “Earth’s first stable continents did not form by subduction”, Nature, Vol 543, pp. 239-242.