On the relative temperatures of Earth's volcanic hotspots and mid-ocean ridges

Volcanic hotspots are thought to be fed by hot, active upwellings from the deep mantle, with excess temperatures (T-ex) similar to 100 degrees to 300 degrees C higher than those of mid-ocean ridges. However, T-ex estimates are limited in geographical coverage and often inconsistent for individual hotspots. We infer the temperature of oceanic hotspots and ridges simultaneously by converting seismic velocity to temperature. We show that while similar to 45% of plume-fed hotspots are hot (T-ex >= 155 degrees C), similar to 15% are cold (T-ex <= 36 degrees C) and similar to 40% are not hot enough to actively upwell (50 degrees C <= T-ex <= 136 degrees C). Hot hotspots have an extremely high helium-3/helium-4 ratio and buoyancy flux, but cold hotspots do not. The latter may originate at upper mantle depths. Alternatively, the deep plumes that feed them may be entrained and cooled by small-scale convection.