To test for the prevalence of mantle plumes and the existence of mantle layering, temperatures (T) are estimated for 28 oceanic hotspots, using olivine-liquid equilibria (Tol-liq). There are 27 localities that have Tol-liq hotter than mid-ocean ridges (MOR), by 99–233 °C (average = 146 ± 26 °C), which translates to mantle potential temperatures that exceed those of MOR by 114–290 °C (average = 173 ± 38 °C). Thermally driven mantle plumes are thus common, not rare. Moreover, mantle temperatures at ocean islands are positively correlated with buoyancy flux and 3He/4He. The correlation with buoyancy affirms that oceanic swells are thermal in origin. The positive correlation with 3He/4He is inconsistent with the notion that high 3He/4He and depleted MOR mantle derive from the same layer, but instead shows that high 3He/4He is tied to a lower thermal boundary layer, and thus that the mantle is compositionally layered. Mantle temperatures are negatively correlated with Pb isotope ratios, supporting a model by C. Class and S.L. Goldstein that this deep, high 3He/4He layer may be depleted.