Chapter 7: On the formation of continental silicic melts in thermo-chemical mantle convection models: implications for early Earth and Venus

Important constituents of Archean cratons, formed in the early and hot history of the Earth, are TTG plutons and greenstone belts. The formation of these granite-greenstone terrains is often ascribed to plate-tectonic processes. Buoyancy considerations, however, do not allow plate tectonics to take place in a significantly hotter Earth. We therefore propose an alternative mechanism for the coeval and proximate production of TTG plutons and greenstone-like crustal successions: When a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle, the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event, in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years. Pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust. Partial melting at the base of this newly produced crust may generate felsic melts that are added as intrusives and/or extrusives to the generally mafic crustal succession, adding to what resembles a greenstone belt. Partial melting of metabasalt in the sinking crustal section produces a significant volume of TTG melt that is added to the crust directly above the location of `subduction', presumably in the form of a pluton. This scenario is self-consistently produced by numerical thermo-chemical mantle convection models, presented in this paper, including partial melting of mantle peridotite and crustal (meta-)basalt. Other geodynamical settings which we have also investigated, including partial melting in small scale delaminations of the lower crust, at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behaviour unequivocally and mostly show melting of metabasalt in the eclogite stability field instead. The resurfacing scenario may also have been important in Venus' history, but probably did not produce significant volumes of continental material due to the dryness of this planet.

This chapter has been submitted by P. van Thienen, A.P. van den Berg and N.J. Vlaar for publication in Tectonophysics