On the dynamics of long-lived plume conduits in the convecting mantle

Abstract

Long-lived plumes, responsible for mantle hotspots and associated chains of volcanism, will be deflected from the vertical as they penetrate through a mantle which is overturning on much larger length scales in response to surface cooling. Recent experiments have shown that plume inclination leads to entrainment of surrounding material as a result of coupling between conduction of heat and laminar stirring. We develop an analysis for entrainment and its effects on the structure and dynamics of continuing, steady conduits in a mantle undergoing horizontal shear. It is found that the stirring of surrounding mantle with plume source material can result in major modifications of the diameter, temperature and mass flux in plumes. This model is a departure from the axisymmetric vertical pipe models of plumes and predicts that the temperature in a defected plume will decrease with height, thus reducing the plume temperature in the upper mantle while allowing a sizeable temperature difference at the source. The cooling of plume conduits, after they are deflected by plate motion, can lead to a lower maximum temperature and lower MgO contents of basalts along some linear hotspot tracks compared with picrites from their associated flood basalt provinces. The effects of entrainment depend primarily on the buoyancy (or heat) flux carried by the plume, with greater cooling for smaller fluxes. Strong continuing plumes such as Hawaii, Easter Island, Macdonald, Tahiti and Reunion are predicted to produce picrites having MgO contents comparable to the picrites found in continental flood basalt provinces. Weak plumes such as Crozet, Bouvet and Juan de Fuca will give only cooler melts. Through entrainment long-lived plumes can sample the whole depth of the mantle from the source to the surface, and stirring within the plume leads to compositional zonation which may contribute to compositional heterogeneity in hotspot melts. We also predict that plumes originating at the core-mantle boundary and having buoyancy fluxes less than about 3 × 10 3 N s −1 will not give surface hotspots, in good agreement with the lower limit of hotspot buoyancy fluxes inferred from seafloor topography.