Melting of the mantle past and present: isotope and trace element evidence

Abstract

Estimates are made of the abundances of some lithophile trace elements, particularly heat-producing elements, in the bulk Earth. The applicability of abundance estimates based on extra-terrestrial analogues, and terrestrial heat flow data are discussed. Sr, Nd and Pb isotope data are briefly reviewed and used to identify basalt source regions in the mantle which have been depleted or enriched in these and other lithophile trace elements. An assessment is made of the role of silicate liquid transfer in the production of depleted mantle. The timing of the transfer event(s) can be constrained using Rb-Sr, Sm-Nd and U-Pb isotope data and cover the period of Earth history during which granitic crust has been stabilized. Calculations of the heat production in the source regions of mid-ocean ridge and other basalts suggest that the convective processes involved in the generation of oceanic lithosphere are driven mainly by heating from beneath, as the internal heat generation is comparatively small. Trace element data from Archaean to Recent volcanics are used to estimate maximum limits on the amount of mantle melting which has occurred in the last 3.5 Ga.