Aluminum-in-olivine thermometry of primitive basalts: Evidence of an anomalously hot mantle source for large igneous provinces

Abstract

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 Fo91.3) 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; Fo91.3) and East Greenland (up to 1354°C; Fo90.1), Cretaceous (86Ma) picrites from Madagascar (up to 1486°C; Fo91.8) and Cretaceous (88Ma) komatiites from Gorgona (up to 1435°C; Fo91.7). Unlike olivine-melt based thermometers these temperature estimates are not significantly influenced by uncertainty in melt H2O content or fO2. 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.