The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35 kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4,5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6,7,8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.
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Condie, K. C. & Pease, V. When Did Plate Tectonics Begin On Planet Earth? 7th edn (Geol. Soc. Am., 2008)
Johnson, T. E., Brown, M., Kaus, B. J. P. & VanTongeren, J. A. Delamination and recycling of Archaean crust caused by gravitational instabilities. Nat. Geosci. 7, 47–52 (2013)
Rapp, R. P., Shimizu, N. & Norman, M. D. Growth of early continental crust by partial melting of eclogite. Nature 425, 605–609 (2003)
Smithies, R. H., Champion, D. C. & Van Kranendonk, M. J. Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt. Earth Planet. Sci. Lett. 281, 298–306 (2009)
Zegers, T. E. & van Keken, P. E. Middle Archean continent formation by crustal delamination. Geology 29, 1083–1086 (2001)
Herzberg, C., Condie, K. & Korenaga, J. Thermal history of the Earth and its petrological expression. Earth Planet. Sci. Lett. 292, 79–88 (2010)
Herzberg, C. & Rudnick, R. Formation of cratonic lithosphere: an integrated thermal and petrological model. Lithos 149, 4–15 (2012)
Korenaga, J. Urey ratio and the structure and evolution of Earth’s mantle. Rev. Geophys. 46, RG2007 (2008)
Sizova, E., Gerya, T., Stüwe, K. & Brown, M. Generation of felsic crust in the Archean: a geodynamic modeling perspective. Precambr. Res. 271, 198–224 (2015)
Moyen, J. F. The composite Archaean grey gneisses: petrological significance, and evidence for a non-unique tectonic setting for Archaean crustal growth. Lithos 123, 21–36 (2011)
Rapp, R. P. & Watson, E. B. Dehydration melting of metabasalt at 8–32 kbar: implications for continental growth and crust-mantle recycling. J. Petrol. 36, 891–931 (1995)
Foley, S., Tiepolo, M. & Vannucci, R. Growth of early continental crust controlled by melting of amphibolite in subduction zones. Nature 417, 837–840 (2002)
Martin, H. The Archean grey gneisses and the genesis of continental crust. Dev. Precamb. Geol. 11, 205–259 (1994)
Smithies, R. H. The Archaean tonalite-trondhjemite-granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth Planet. Sci. Lett. 182, 115–125 (2000)
Bédard, J. H. A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochim. Cosmochim. Acta 70, 1188–1214 (2006)
Willbold, M., Hegner, E., Stracke, A. & Rocholl, A. Continental geochemical signatures in dacites from Iceland and implications for models of early Archaean crust formation. Earth Planet. Sci. Lett. 279, 44–52 (2009)
Moyen, J. F. & Stevens, G. in Archean Geodynamics and Environments (eds Benn, K., Mareschal, J.-C. & Condie, K. C ) 149–175 (Am. Geophys. Union, 2006)
Martin, H. & Moyen, J.-F. Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth. Geology 30, 319–322 (2002)
Green, E. C. R. et al. Activity-composition relations for the calculation of partial melting equilibria in metabasic rocks. J. Metamorph. Geol. 34, 845–869 (2016)
Van Kranendonk, M. J., Hickman, A. H., Smithes, R. H. & Nelson, D. R. Geology and tectonic evolution of the Archean North Pilbara Terrain, Pilbara Craton, Western Australia. Econ. Geol. 97, 695–732 (2002)
Van Kranendonk, M. J. et al. Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons. Geol. Soc. Spec. Publ. 389, 83–111 (2015)
Kerrich, R., Polat, A., Wyman, D. & Hollings, P. Trace element systematics of Mg-, to Fe-tholeiitic basalt suites of the Superior Province: implications for Archean mantle reservoirs and greenstone belt genesis. Lithos 46, 163–187 (1999)
Condie, K. C. Incompatible element ratios in oceanic basalts and komatiites: tracking deep mantle sources and continental growth rates with time. Geochem. Geophys. Geosyst. 4, 1–28 (2003)
Berry, A. J. et al. Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle. Nature 455, 960–963 (2008)
Brown, M. The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics. Geosci. Frontiers 5, 553–569 (2014)
Xiong, X. et al. Partitioning of Nb and Ta between rutile and felsic melt and the fractionation of Nb/Ta during partial melting of hydrous metabasalt. Geochim. Cosmochim. Acta 75, 1673–1692 (2011)
McKenzie, D. & O’Nions, R. K. Partial melt distributions from inversion of rare earth element concentrations. J. Petrol. 32, 1021–1091 (1991)
Sun, S.-s. & McDonough, W. F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geol. Soc. Spec. Publ. 42, 313–345 (1989)
Powell, R. & Holland, T. J. B. An internally consistent dataset with uncertainties and correlations: 3. Applications to geobarometry, worked examples and a computer program. J. Metamorph. Geol. 6, 173–204 (1988)
Holland, T. J. B. & Powell, R. An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids. J. Metamorph. Geol. 29, 333–383 (2011)
White, R. W., Powell, R., Holland, T. J. B., Johnson, T. E. & Green, E. C. R. New mineral activity-composition relations for thermodynamic calculations in metapelitic systems. J. Metamorph. Geol. 32, 261–286 (2014)
White, R. W., Powell, R. & Clarke, G. L. The interpretation of reaction textures in Fe-rich metapelitic granulites of the Musgrave Block, Central Australia: constraints from mineral equilibria calculations in the system. J. Metamorph. Geol. 20, 41–55 (2002)
White, R., Powell, R., Holland, T. & Worley, B. The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 . J. Metamorph. Geol. 18, 497–511 (2000)
Holland, T. & Powell, R. Activity–composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contrib. Mineral. Petrol. 145, 492–501 (2003)
Boehnke, P., Watson, E. B., Trail, D., Harrison, T. M. & Schmitt, A. K. Zircon saturation re-revisited. Chem. Geol. 351, 324–334 (2013)
Stimac, J. & Hickmott, D. Trace-element partition coefficients for ilmenite, orthopyroxene and pyrrhotite in rhyolite determined by micro-PIXE analysis. Chem. Geol. 117, 313–330 (1994)
We acknowledge financial support from The Institute of Geoscience Research (TIGeR) at Curtin University. R.H.S. publishes with the permission of the Executive Director, Geological Survey of Western Australia.
The authors declare no competing financial interests.
Reviewer Information Nature thanks J. Bédard, R. Rapp and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
a, La/Nb versus Ti/Gd ratios. b, La/Nb ratios versus MgO (wt%) concentration. c, La/Nb ratios versus wt% silica (SiO2, anhydrous) concentrations. d, Th/Nb ratios versus wt% SiO2 (anhydrous) concentrations. Five samples (green dots) that define a trend to higher La/Nb and Th/Nb ratios with increasing SiO2 levels and decreasing Ti/Gd ratios are thought to have undergone assimilation of felsic crust4 and were excluded from subsequent modelling. The remaining ten samples (pink dots), with low Th/Nb and La/Nb ratios, are considered to be free of major crustal influence22,23.
Full pressure–temperature (P–T) pseudosection for the average uncontaminated Coucal basalt (n = 10), using an Fe3+/ΣFe ratio of 0.1 (ref. 24) and a water content just sufficient to saturate the solidus at 1.0 GPa. The red dashed lines show melt proportions as mol% on a one-oxide basis to approximate vol%. The white dashed lines show linear geotherms in °C per gigapascal. The 900 °C GPa−1 geotherm discussed in detail in the text, is shown in yellow. This diagram forms the basis for the simplified version shown in Fig. 2. aug, augite; bi, biotite; g, garnet; hb, hornblende; H2O, aqueous fluid; ilm, ilmenite; ksp, K-feldspar; L, melt; mt, magnetite; muscovite (mu); opx, orthopyroxene; pl, plagioclase; q, quartz; ru, rutile; sph, sphene = titanite.
Extended Data Figure 3 Apparent geotherms recorded in metamorphic rocks dated to 2.4 Gyr ago or older.
The geotherms (T/P) are shown in units of °C per gigapascal. All but one rock of Archaean age (2.5 Gyr or older) record apparent geotherms of 500 °C GPa−1 or higher, and most are more than, or much more than, 700 °C GPa−1 (M.B. and T.E.J., manuscript in preparation). These findings are consistent with a global database of TTG compositions, 76% of which are LP- and MP-TTGs10 (see yellow and green fields in Fig. 2).
Extended Data Figure 4 Calculated melt compositions for melting along apparent geotherms of 700 °C GPa−1 and 1,100 °C GPa−1.
Calculated composition of melt produced from the average Coucal basalt along linear geotherms of 700 °C GPa−1 (top) and 1,100 °C GPa−1 (bottom) at various melt fractions (5–40%), normalized to the average composition of Palaeoarchaean East Pilbara TTG. The grey shaded region shows the 2σ envelope on the average Palaeoarchaean East Pilbara TTG data. Also shown are the average composition of TTGs and potassic granitoids worldwide10. As with the 900 °C GPa−1 data (Fig. 3a), the modelled melt compositions fit best at melt fractions of 20–30 mol% (approximate vol%).
Extended Data Figure 5 Calculated modal proportion for melting along apparent geotherms of 700 °C GPa−1 and 1,100 °C GPa−1.
Plots of modal proportions versus temperature (also known as modebox diagrams) showing the changing abundance of phases, modelled along linear geotherms of 700 °C GPa−1 (top) and 1,100 °C GPa−1 (bottom). Assemblages developed along the cooler geotherm (700 °C GPa−1) contain (at temperatures greater than 700 °C) abundant garnet, as well as a small quantity (<2 mol%) of suprasolidus sphene (titanite) at lower temperatures (less than 750 °C), of ilmenite at intermediate temperatures (700–860 °C), and of rutile at temperatures higher than 800 °C (that is, these assemblages are HP-TTGs). Little garnet and no rutile forms along the 1,100 °C GPa−1 geotherm, consistent with the formation of LP- and MP-TTGs; all of these assemblages contain a small amount (less than 2 mol%) of ilmenite, but no sphene.
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Johnson, T., Brown, M., Gardiner, N. et al. Earth’s first stable continents did not form by subduction. Nature 543, 239–242 (2017). https://doi.org/10.1038/nature21383
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