Delamination and recycling of Archaean crust caused by gravitational instabilities

摘要

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 mantle~1. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle~2. Furthermore, Archaean crust exposed today is composed mostly of tonalite– trondhjemite–granodiorite, indicative of a hydrated, lowmagnesium basalt source~3, 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.