Upwellings from a deep mantle reservoir filtered at the 660Â km phase transition in thermo-chemical convection models and implications for intra-plate volcanism

With numerical modelling experiments we examine the influence of the 660 km post-spinel phase transition on the excess temperatures in and transport of compositional heterogeneity by deep mantle upwellings. We test our results against available observations on ocean island basalts (OIB). The endothermic post-spinel transition can cause localised, transient layering of convective flow in the upper mantle transition zone. Upwellings from the lower mantle, in our models the passive (mechanically driven) return flow from active (buoyancy-driven) downwellings, can transport heterogeneous material from deep mantle reservoirs across the transition zone into the upper mantle, but the phase transition can act as a chemical filter that can (locally) retain intrinsically denser elements in the lower mantle. A compositionally heterogeneous signature in upwellings from the lower mantle (e.g. Hofmann, A.W., 1997. Mantle geochemistry: the message from oceanic volcanism. Nature 385, 219-229) that is consistent with chemical diversity in OIB can be produced for a wide range of mantle evolution scenarios and stability conditions of compositional heterogeneity in the lower mantle. We demonstrate that the excess temperature in upwellings crossing the phase boundary increases with the background temperature contrast across the endothermic phase transformation, which itself increases with the degree of convective layering. For cases with a low degree of layering at the transition zone, the combination of upwellings of mid-mantle origin and intrinsically hot, deep-seated upwellings produce model results consistent with the formation of hotspots and large igneous provinces (LIP) at the surface. For high degrees of convective layering, however, lower mantle upwellings of high excess temperature would generate (with high frequency) massive flood basalt events. In view of the strong influence of transition zone convective layering on excess temperatures, the observed intra-plate volcanism in modern Earth suggests that on a global scale the degree of layering at the transition zone is low, which requires a low Clapeyron slope magnitude of the post-spinel phase transition. Because hot intrinsically dense material residing below the phase transition favours a high degree of convective layering, the Clapeyron slope magnitude must be even lower for this case in order to produce results consistent with inferences from mantle petrology.