An impact melt origin for Earth’s oldest known evolved rocks


Earth’s oldest evolved (felsic) rocks, the 4.02-billion-year-old Idiwhaa gneisses of the Acasta Gneiss Complex, northwest Canada, have compositions that are distinct from the felsic rocks that typify Earth’s ancient continental nuclei, implying that they formed through a different process. Using phase equilibria and trace element modelling, we show that the Idiwhaa gneisses were produced by partial melting of iron-rich hydrated basaltic rocks (amphibolites) at very low pressures, equating to the uppermost ~3 km of a Hadean crust that was dominantly mafic in composition. The heat required for partial melting at such shallow levels is most easily explained through meteorite impacts. Hydrodynamic impact modelling shows not only that this scenario is physically plausible, but also that the region of shallow partial melting appropriate to formation of the Idiwhaa gneisses would have been widespread. Given the predicted high flux of meteorites in the late Hadean, impact melting may have been the predominant mechanism that generated Hadean felsic rocks.

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Fig. 1: Major element oxide and trace element geochemistry.
Fig. 2: Phase equilibria modelling.
Fig. 3: Trace element modelling.
Fig. 4: Hydrocode numerical simulation.


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T.E.J. acknowledges financial support from the State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (Open Fund GPMR210704), and from the Office of Research and Development (ORD) and The Institute of Geoscience Research (TIGeR), Curtin University. K.M. acknowledges Australian Research Council (ARC) funding and the developers of the iSALE hydrocode. H.S. publishes with the permission of the Executive Director, Geoscience and Resource Strategy. P.A.B. acknowledges support from ARC DP170102529.

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T.E.J. conceived the idea for the paper and did the phase equilibria modelling. T.E.J. and N.J.G. undertook the trace element modelling. K.M. performed the hydrodynamic impact modelling. T.E.J. wrote the manuscript draft. All authors contributed to discussions and the writing of the final paper.

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Correspondence to Tim E. Johnson.

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Johnson, T.E., Gardiner, N.J., Miljković, K. et al. An impact melt origin for Earth’s oldest known evolved rocks. Nature Geosci 11, 795–799 (2018).

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