Modern oceanic crust is constantly produced at oceanic ridges and recycled back into the mantle at subduction zones via plate tectonics. An outstanding question in geology is whether the Earth started in a non-plate tectonic regime, and if it did, when the transition to the modern regime occurred. This is a complicated question to address because Archaean rocks lack modern equivalents to anchor interpretations. Here, we present a silicon isotopic study of 4.0–2.8-Gyr-old tonalite–trondhjemite–granodiorites, as well as Palaeozoic granites and modern adakites. We show that Archaean granitoids have heavier silicon isotopic compositions than granites and adakites, regardless of melting pressure. This is best explained if Archaean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater. Before the appearance of silica-forming organisms 0.5–0.6 billion years ago, the oceans were close to silicon saturation, which led to extensive precipitation of cherts on the seafloor. This is in contrast to modern oceans, where silica biomineralization maintains dissolved silicon at low concentration. The unique heavy silicon isotope signature of cherts has been transferred to Archaean granitoids during an oceanic subduction process, which was probably responsible for the formation of felsic rocks on Archaean emerged lands.
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The authors declare that the data supporting the findings of this study are available within the article and its supplementary information files (that is, the Supplementary Information and additional datasets).
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We thank P. Louvat, T. Sontag and P. Burckel for help with the (multi-collector) inductively coupled plasma mass spectrometry. We thank G. Libourel for suggestions. I. Bindeman, M. Krawczynski, F. Poitrasson, E. J. Chin and M. Harrison are appreciated for their comments on an earlier version of this manuscript. I.S.P.’s komatiite sample collection benefited from contributions of E. Nisbet, G. Byerly and C. Anhaeusser. We thank O. Sigmarsson for providing adakite samples. F.M. acknowledges funding from the ERC under the H2020 framework programme/ERC grant agreement no. 637503 (Pristine). F.M. and M.C. thank the financial support of the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP platform PARI, and by Region Île-de-France Sesame grant no. 12015908. M.G. acknowledges financial support from Région Auvergne (Auvergne Fellowship programme), LabEx ClerVolc (ANR-10-LABX-0006) and Agence National de la Recherche (ANR-17-CE31-0021 Zircontinents). This is ClerVolc contribution 353.
The authors declare no competing interests.
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Supplementary information on samples and calculations and additional figures (Supplementary Figs. 1–5).
Chemical and silicon isotopic compositions.
Silicon isotopic data for terrestrial samples in literature.
Primitive mantle normalized values for TTGs.
Modelling silicon isotopic fractionations between minerals and melts.
Modelling strontium and silicon of felsic melts.
Summary of bulk oxygen isotopic data for Archaean TTGs.
Whole-rock oxygen isotopic data.
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Deng, Z., Chaussidon, M., Guitreau, M. et al. An oceanic subduction origin for Archaean granitoids revealed by silicon isotopes. Nat. Geosci. 12, 774–778 (2019). https://doi.org/10.1038/s41561-019-0407-6
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