Geochemical Constraints on Mantle Melting during Creation of the North-Atlantic Basin

THE compositions of magmas produced by decompression melting of upwelling mantle rocks are sensitive to the extent and mean pressure of melting; these, in turn, depend respectively on the depth at which the solidus is encountered1,2 and on the thickness of the lithosphere, which provides a barrier to upwelling mantle2,3. Here we report major- and trace-element data for lavas erupted during rifting of the Greenland-European continent approximately 60 Myr ago, which show a trend to higher extents of melting at lower pressures as rifting proceeded. We attribute these changes to progressive thinning of the continental lithosphere during the initial phase of magmatism. Our analysis also shows that mantle melting began well within the garnet stability field, supporting previous suggestions4-8 that anomalously hot mantle was present beneath the region at the time of rifting. The modest extents of melting that we infer for the earliest rift lavas can largely account for their high contents of incompatible elements, thus reducing the degree of geochemical enrichment ('plume-like' character) required in the mantle source region.