Delamination and recycling of Archaean crust caused by gravitational instabilities

Abstract

Mantle temperatures during the Archaean eon were higher than today. As a consequence, the primary crust formed at the time is thought to have been extensive, thick and magnesium rich, and underlain by a highly residual mantle1. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle2. Furthermore, Archaean crust exposed today is composed mostly of tonalite–trondhjemite–granodiorite, indicative of a hydrated, low-magnesium basalt source3, suggesting that they were not directly generated from a magnesium-rich primary crust. Here we present thermodynamic calculations that indicate that the stable mineral assemblages expected to form at the base of a 45-km-thick, fully hydrated and anhydrous magnesium-rich crust are denser than the underlying, complementary residual mantle. We use two-dimensional geodynamic models to show that the base of magmatically over-thickened magnesium-rich crust, whether fully hydrated or anhydrous, would have been gravitationally unstable at mantle temperatures greater than 1,500–1,550 °C. The dense crust would drip down into the mantle, generating a return flow of asthenospheric mantle that melts to create more primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to source tonalite–trondhjemite–granodiorite melts. The residues of these processes, with an ultramafic composition, must now reside in the mantle.

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Figure 1: Calculated primary melt compositions for Precambrian non-arc basalts.
Figure 2: Results of thermodynamic modelling of primary crust and complementary residues at 1,000 °C.
Figure 3: Density (ρ) of primary crust and complementary residues.
Figure 4: Results of geodynamic modelling; snapshots from an experiment with 45-km-thick initial primary crust and Tp of 1,600 °C.

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Acknowledgements

We thank S. Aulbach, J. Connolly, G. Davies, S. Fischer, S. F. Foley, E. C. R. Green, C. Herzberg, D. E. Jacob and R. W. White for comments. M.B. and T.E.J. acknowledge financial support from the Geocycles Earth Systems Research Centre, University of Mainz. B.J.P.K. was financially supported by ERC Starting Grant 258830.

Author information

M.B. and T.E.J. developed the project; T.E.J. calculated the phase diagrams and B.J.P.K. developed and ran the numerical models. All authors discussed the results and were involved in writing the paper.

Correspondence to Tim E. Johnson.

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The authors declare no competing financial interests.

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Johnson, T., Brown, M., Kaus, B. et al. Delamination and recycling of Archaean crust caused by gravitational instabilities. Nature Geosci 7, 47–52 (2014). https://doi.org/10.1038/ngeo2019

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