Abstract
Much of the current volume of Earth’s continental crust had formed by the end of the Archaean eon1 (2.5 billion years ago), through melting of hydrated basaltic rocks at depths of approximately 25–50 kilometres, forming sodic granites of the tonalite–trondhjemite–granodiorite (TTG) suite2,3,4,5,6. However, the geodynamic setting and processes involved are debated, with fundamental questions arising, such as how and from where the required water was added to deep-crustal TTG source regions7,8. In addition, there have been no reports of voluminous, homogeneous, basaltic sequences in preserved Archaean crust that are enriched enough in incompatible trace elements to be viable TTG sources5,9. Here we use variations in the oxygen isotope composition of zircon, coupled with whole-rock geochemistry, to identify two distinct groups of TTG. Strongly sodic TTGs represent the most-primitive magmas and contain zircon with oxygen isotope compositions that reflect source rocks that had been hydrated by primordial mantle-derived water. These primitive TTGs do not require a source highly enriched in incompatible trace elements, as ‘average’ TTG does. By contrast, less sodic ‘evolved’ TTGs require a source that is enriched in both water derived from the hydrosphere and also incompatible trace elements, which are linked to the introduction of hydrated magmas (sanukitoids) formed by melting of metasomatized mantle lithosphere. By concentrating on data from the Palaeoarchaean crust of the Pilbara Craton, we can discount a subduction setting6,10,11,12,13, and instead propose that hydrated and enriched near-surface basaltic rocks were introduced into the mantle through density-driven convective overturn of the crust. These results remove many of the paradoxical impediments to understanding early continental crust formation. Our work suggests that sufficient primordial water was already present in Earth’s early mafic crust to produce the primitive nuclei of the continents, with additional hydrated sources created through dynamic processes that are unique to the early Earth.
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Data availability
Oxygen isotope data for this study were collected on 39 samples and data on two other samples were taken from ref. 22 (sample OGC; GSWA sample 178035) and ref. 53 (sample 142661). All sample details, oxygen isotope and U–Pb age data from these samples are presented in Extended Data Tables 1, 2 (and Supplementary Tables 1–3) and whole-rock geochemical data are presented in Supplementary Table 4. Raw data relating to U–Pb age determinations can be downloaded from ref. 57. Whole-rock geochemistry data for Pilbara and Yilgarn sanukitoids are from refs. 36,44, respectively. Data used to calculate average compositions of Pilbara basalts are from ref. 56. Data in refs. 36,44,56 can also be downloaded, using sample numbers provided as Supplementary Table 6, from the Geological Survey of Western Australia’s WACHEM database (http://geochem.dmp.wa.gov.au/geochem/) using the GeoChem Extract tool and selecting the ‘Whole State’ option, and from Geoscience Australia’s OZCHEM National Whole Rock Geochemistry Dataset (http://pid.geoscience.gov.au/dataset/ga/65464). Whole-rock geochemistry data used in Fig. 1 and used to calculate average data for trace-element modelling (presented in Fig. 3) and from sources cited in ref. 35 or can be downloaded either from WACHEM or OZCHEM using sample numbers provided as Supplementary Table 6. Source data are provided with this paper.
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Acknowledgements
R.H.S., Y.L., M.T.D.W. and S.P.J. publish with the permission of the Executive Director, Geological Survey of Western Australia. T.E.J. acknowledges funding from the Australian Government through an Australian Research Council Discovery Project (DP200101104), and support from the State Key Laboratory for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (Open Fund GPMR201903). This project was supported by funding from the Western Australian Government Exploration Incentive Scheme (EIS). We thank M. Prause for drafting the figures and M. Aleshin for assistance with SIMS analyses. We acknowledge the facilities, and the scientific and technical assistance, of Microscopy Australia at the Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, a facility funded by the University, and the Western Australian and Commonwealth of Australia governments.
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R.H.S. and Y.L. conceived the research, and R.H.S., Y.L., C.L.K. and T.E.J. prepared the first draft of the manuscript with input from D.R.M. and D.C.C. L.M. and H.J. contributed to sample analyses and interpretation and M.T.D.W. and S.P.J. contributed to sample collection and data interpretation. All co-authors assisted in reviewing and editing the revised manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Location maps for samples.
a, Samples taken from the Pilbara Craton. WPT, West Pilbara Terrane; CPTZ, Central Pilbara Tectonic Zone; MCB, Mosquito Creek Basin. b, Samples taken from the Yilgarn Craton. The geological map is from ref. 58, reproduced under a CC BY 4.0 licence.
Extended Data Fig. 2 Compositions of potential hybrid sources.
Major element compositions of potential hybrid source regions normalized against average >3.45-Gyr-old Pilbara tholeiitic basalt. Av., average.
Extended Data Fig. 3 Oxygen isotope compositions of Pilbara and Yilgarn granites.
a, Variation in O isotope compositions of zircon (δ18O(zircon)) from granites of the Pilbara Craton against whole-rock SiO2 and the zircon saturation temperature, Tzircon saturation55. b, Variation in O isotope compositions of zircon (δ18O(zircon)) from granites of the Yilgarn Craton against whole-rock SiO2 and zircon saturation temperature55. The dashed horizontal lines indicate the 2σ range of mantle zircon, 5.3‰ ± 0.6‰. Vertical error bars represent 1σ uncertainty.
Extended Data Fig. 4 Variation of Sr/Sr* with SiO2 for sanukitoids from the Pilbara Craton36.
Values above or below one indicate positive or negative mantle-normalized Sr anomalies, respectively. Sr* is the Sr concentration calculated by extrapolating between Ce and Nd on mantle-normalized trace-element diagram.
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Smithies, R.H., Lu, Y., Kirkland, C.L. et al. Oxygen isotopes trace the origins of Earth’s earliest continental crust. Nature 592, 70–75 (2021). https://doi.org/10.1038/s41586-021-03337-1
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DOI: https://doi.org/10.1038/s41586-021-03337-1
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