The maximum crystallization temperature of mantle-derived melts provides constraints on the minimum temperature of their mantle source regions. This temperature can be estimated using phenocryst-based thermometry on primitive lavas that contain olivine and spinel phenocrysts using the Al-in-olivine thermometer. Application of an updated version of this thermometer to primitive basalts gives precise estimates of olivine-spinel crystallization temperatures in different geodynamic settings. These are minimum temperatures for melt extraction from the mantle due to both adiabatic cooling and cooling prior to co-saturation in these phases. Application of this thermometer to primitive MORB gives co-saturation temperatures of up to 1270°C (and a maximum olivine forsterite (Fo) content of Fo 91.3 ) consistent with previous constraints (e.g. from the volume of melt produced at mid-ocean ridges). Substantially higher crystallization temperatures are recorded by Tertiary (~60Ma) picrites from Baffin Island (up to 1408°C; Fo 91.3 ) and East Greenland (up to 1354°C; Fo 90.1 ), Cretaceous (86Ma) picrites from Madagascar (up to 1486°C; Fo 91.8 ) and Cretaceous (88Ma) komatiites from Gorgona (up to 1435°C; Fo 91.7 ). Unlike olivine-melt based thermometers these temperature estimates are not significantly influenced by uncertainty in melt H 2 O content or fO 2 . These results indicate that the mantle source region of large igneous provinces is substantially hotter than the ambient upper mantle, supporting the standard thermal plume model.