MELT GENERATION DURING RIFTING IN THE NORTH-SEA

IN a forthcoming paper, McKenzie and O'Nions 1 invert rare-earth element concentrations in mid-ocean-ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges. They find that melting persists to greater depths (about 80 km) than would be predicted by parameterizations of melting experiments 2 (about 50 km). This result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting. The North Sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained 3-8. Using the distribution of melt fraction with depth derived in ref. 1, we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the North Sea (the Forties region). When this melt is mixed in approximately equal proportions with an alkaline melt, similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks, it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region.