Evidence from the rare-earth-element record of mantle melting for cooling of the tertiary Iceland plume

Widespread flood basalt volcanism and continental rifting in the northeast Atlantic in the early Tertiary period (similar to 55 Myr ago) have been linked to the mantle plume now residing beneath Iceland(1-5) Although much is known about the present-day Iceland plume(6-9), its thermal structure, composition and position in the early Tertiary period remain unresolved. Estimates of its temperature, for example, range from >1,600 degrees C in some plume models(3) to similar to 1,500 degrees C based on the volume and composition of basaltic crust(10-12). Several recent studies(4) have emphasized similarities in the thermal and chemical structure of the Tertiary and present-day plumes to argue for stability of the mantle anomaly, whereas others(12,13) relate variations in basalt volumes and compositions to changes in plume flux. Moreover, some authors(1,2,13) have assumed that the plume was rift-centred for its entire history, whereas others argue that it became ridge-centred only after plate separation(14,15), Here we report compositional data for similar to 6,000 metres of flood basalts erupted in east Greenland, dose to the inferred plume axis, that we use to constrain the Tertiary plume structure. Rare-earth-element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot (similar to 1,500 degrees C), but that its temperature declined during flood volcanism. These observations are difficult to reconcile with current plume-head models, and call for important lithospheric control(5,10,16-18) on actively upwelling mantle along the rifted margin.