Abstract

Decompression melting of hot mantle rising in the convectively driven core of the Iceland plume generates igneous crust beneath Iceland that is c. 25 km thick. Passive decompression of mantle beneath the adjacent Reykjanes Ridge spreading centre produces crust 7–10 km thick. The decrease between Iceland and the adjacent oceanic spreading centre in crustal thickness, in basement elevation and in melting column thickness deduced from rare earth element inversions of basaltic igneous rocks, suggests that the mantle beneath the spreading centre is markedly cooler (potential temperature less than 1400°C) than in the core of the plume under Iceland (potential temperature c. 1500 °C). We suggest that the spreading centre is fed by mantle from a cooler sheath surrounding the narrow central core of the plume under Iceland, and that the mantle cools to the south until it reaches a normal potential temperature of 1300°C about 1350 km away from the centre of the plume. Prominent V-shaped topography and gravity anomalies on the Reykjanes Ridge record variations of up to 30°C in the temperature of the underlying asthenospheric mantle supplied by the plume, over timescales of 5–10 Ma.