The mantle components that represent the source region of ocean island basalts (OIB) and feed hotspot volcanism are predicted to contain 160 ± 20 (2σm) ng/g Th, a heat-producing element. This critical model composition indicates that the OIB source region (OSR) comprises a significant amount of recycled oceanic crust and constitutes 19+3−2 (2σm)% of the mantle by mass. The mass fraction of this reservoir supports a mantle architecture with a basal thermochemical layering at an average depth of 2000 ± 100 (2σm) km or two thermochemical piles that extend up to midmantle levels. The hotspot source described here generates 10 pW/kg of radiogenic heat and supplies 7.3 TW to the planet's total surface heat flux. Given that the silicate portion of the Earth produces some 20.4 TW of radiogenic power, with 7.2 TW derived from the continental crust, the mantle source responsible for mid-ocean ridge volcanism provides only 5.9 TW of radiogenic power (or <2 pW/kg). As a result, the source of hotspots generates >5x more radiogenic heat than the source of mid-ocean ridges, thus contributing to the energetics that drive mantle convection and potentially the formation of long-lived plumes via bottom heating of the modern mantle. The potential for a sequestered or unsampled mantle reservoir would impact the relative mass fractions of the source regions of OIB and mid-ocean ridge volcanism but not the compositional model of the OSR presented here.