THE MELTING PROCESSES AND COMPOSITION OF THE NORTH-ATLANTIC (ICELAND) PLUME - GEOCHEMICAL EVIDENCE FROM THE EARLY TERTIARY BASALTS

Three successive geochemical magma types, which result from the decompression melting of the Iceland plume, can be identified throughout the North Atlantic Tertiary Igneous Province. Compared with Type 1, the geochemical signature of Type 2 can be interpreted in terms of shallower, more extensive melting, in response to lithospheric thinning. The chemistry of the youngest lavas and dykes (Type 3) suggests small degree melting at greater mantle depths than Type 2, perhaps below a lithospheric lid which had been re-thickened by underplating. The depleted (relative to Bulk Earth) geochemical signatures of Types 1 and 2 appear not to be derived from entrained upper MORB-source mantle, nor an immature HIMU plume. Rather, it is proposed that they are the melting products of an intrinsically depleted plume.