The newly developed Al-in-olivine geothermometer was used to find the olivine-Cr-spinel crystallization temperatures of a suite of picrites spanning the spatial and temporal extent of the North Atlantic Igneous Province (NAIP), which is widely considered to be the result of a deep-seated mantle plume. Our data confirm that start-up plumes are associated with a pulse of anomalously hot mantle over a large spatial area before becoming focused into a narrow upwelling. We find that the thermal anomaly on both sides of the province at Baffin Island/West Greenland and the British Isles at $61 Ma across an area $2000 km in diameter was uniform, with Al-in-olivine temperatures up to $300 C above that of average mid-ocean ridge basalt (MORB) primitive magma. Furthermore, by combining our results with geochemical data and existing geophysical and bathymetric observations, we present compelling evidence for long-term (>10 7 year) fluctuations in the temperature of the Iceland mantle plume. We show that the plume temperature fell from its initial high value during the start-up phase to a minimum at about 35 Ma, and that the mantle temperature beneath Iceland is currently increasing.

The viscosity of liquid fayalite (Fe 2 SiO 4 ) was determined up to 9.2 GPa and 1850°C using in situ falling sphere viscometry and X-ray radiography imaging. The viscosity of liquid fayalite was found to decrease both along the melting curve and an isotherm, therefore temperature is thought to have little effect on liquid fayalite viscosity at high pressure. The results are in contrast with previous studies on depolymerised silicate melts which found viscosity to increase with pressure. In accordance with recent in situ structural measurements on liquid fayalite, the viscosity decrease is likely a result of the increase in Fe-O coordination with pressure. The results show that liquid silicate viscosities need to be considered on an individual basis and can be strongly dependent on the melt structure and composition. This has important implications for models of planetary differentiation. In particular, terrestrial bodies with high Fe contents and reducing mantle conditions are likely to have had very mobile melts at depth.