THE TEMPERATURE OF THE ICELAND PLUME AND ORIGIN OF OUTWARD-PROPAGATING V-SHAPED RIDGES

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 adjace nt 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 thickne ss deduced from rare earth element inversions of basaltic igneous rock s, suggests that the mantle beneath the spreading centre is markedly c ooler (potential temperature less than 1400 degrees C) than in the cor e of the plume under Iceland (potential temperature c. 1500 degrees 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 pote ntial temperature of 1300 degrees C about 1350km away from the centre of the plume. Prominent V-shaped topography and gravity anomalies on t he Reykjanes Ridge record variations of up to 30 degrees C in the temp erature of the underlying asthenospheric mantle supplied by the plume, over timescales of 5-10 Ma.