GEOCHEMICAL CONSTRAINTS ON MANTLE MELTING DURING CREATION OF THE NORTH-ATLANTIC BASIN

THE compositions of magmas produced by decompression melting of upwell ing mantle rocks are sensitive to the extent and mean pressure of melt ing; these, in turn, depend respectively on the depth at which the sol idus is encountered1,2 and on the thickness of the lithosphere, which provides a barrier to upwelling mantle2,3. Here we report major- and t race-element data for lavas erupted during rifting of the Greenland-Eu ropean continent approximately 60 Myr ago, which show a trend to highe r extents of melting at lower pressures as rifting proceeded. We attri bute these changes to progressive thinning of the continental lithosph ere during the initial phase of magmatism. Our analysis also shows tha t mantle melting began well within the garnet stability field, support ing previous suggestions4-8 that anomalously hot mantle was present be neath the region at the time of rifting. The modest extents of melting that we infer for the earliest rift lavas can largely account for the ir high contents of incompatible elements, thus reducing the degree of geochemical enrichment ('plume-like' character) required in the mantl e source region.