Dehydration reactions at the top of the mantle transition zone (MTZ) can stabilize partial melt in a seismic low-velocity layer (LVL), but the seismic effects of temperature, melt and volatile content are difficult to distinguish. We invert P-to-S receiver function phases converted at the top and bottom of a LVL above the MTZ beneath Hawaii. To separate the thermal and melting related seismic anomalies, we carry out over 10 million rock physics inversions. These inversions account for variations arising from the Clapeyron slope of phase transition, bulk solid composition, dihedral angle, and mantle potential temperature. We use two independent seismic constraints to evaluate the temperature and shear wave speed within the LVL. The thermal anomalies reveal the presence of a hot and seismically slow plume stem surrounded by a “halo” of cold and fast mantle material. In contrast to this temperature distribution, the plume stem contains less than 0.5 vol% melt, while the surrounding LVL—within the coverage area—contains up to 1.7 vol% melt, indicating possible lateral transport of the melt. When compared to the melting temperatures of mantle rocks, the temperature within the LVL, calculated from seismic observations of MTZ thickness, suggests that the observed small degrees of melting are sustained by the presence of volatiles such as CO2 and H2O. We estimate the Hawaiian plume loses up to 1.9 Mt/yr H2O and 10.7 Mt/yr CO2 to the LVL, providing a crucial missing flux for global volatile cycles
