Modelled primary magma compositions of Palaeogene basalts from the North Atlantic Igneous Province (NAIP) require melting at mantle potential temperatures (TP) in the range 1480–1550°C. Modern lavas from Icelandic rift zones require TP ∼ 1500°C and those from the rift flanks TP ∼ 1450°C. Secular cooling of the NAIP thermal anomaly was therefore of the order of ∼50°C over the past 61 Myr. There were systematic variations in TP of 50–100°C from the centre of the thermal anomaly to its margins at any one time, although limits on the stratigraphical distribution of TP determinations do not rule out thermal pulsing on a timescale of millions of years. Variation in extent of melting at similar TP was controlled by local variability in lithospheric thickness. In the west of the NAIP, lithosphere thickness varied from ∼90 km at Disko Island to ∼65 km at Baffin Island, with similar thickness variations being evident for magmatism in the Faroe Islands, Faroe–Shetland Basin and the British Palaeogene Igneous Province (BPIP). Mean pressure of melting was greater than or equal to the final pressure of melting; the two values converge for melting columns with a melting interval of <1·5 GPa, regardless of TP. The majority of the BPIP magmas were generated in the garnet–spinel transition in the upper mantle. Calculated and observed rare earth element distributions in NAIP lavas are entirely consistent with the melting regimes derived from major element melting models. This allows a calibration of rare earth element fractionation and melting conditions that can be applied to other flood basalt provinces.