Ultrafast mantle plumes and implications for flood basalt volcanism in the Northern Atlantic region

Recent geochemical and geochronological data on Paleocene flood basalts from West Greenland, SE Greenland and the British Isles show that volcanic activity at these widely separated locations commenced nearly simultaneously around 61-62 Ma ago, and that the duration of this initial phase of flood basalt magmatism was of the order of a few million years or less. A small, fast moving upper mantle plume which rapidly spreads out horizontally, on encountering the base of the lithosphere, appears to be a viable mechanism to explain these observations. However, in order to reconcile the idea of an ultrafast plume with the observed plate velocities, a physical mechanism is needed for inducing a separation of timescales between the plume speed and the surrounding mantle circulation. Thermal convection with a non-Newtonian temperature-and depth-dependent rheology provides such as mechanism wherein extremely fast plumes ascending at velocities between one to tens of meters per year can be produced in an otherwise slowly convecting mantle moving at cm/yr. This transport mechanism is capable of bringing up very hot matter from the transition zone to the lithosphere. The fast upward velocities lead to the production of high viscous heating rates surrounding the plume upon impinging the lithosphere. It is thus possible to explain the near simultaneous onset of magmatism in West Greenland, SE Greenland and the British Isles in the early Tertiary by a fast moving mantle plume spreading out horizontally with a velocity of around 0.5 m/yr. For resolving numerically the thermal-mechanical state of these strongly time-dependent mantle flows, extremely high spatial resolution, on the kilometer scale, is required. Finally we suggest the possibility that the source of the fast upper-mantle plume under Iceland may be rooted in the lower mantle. This is consistent with the recent findings by seismic tomography of a deep mantle plume under Iceland.