 Mantle potential temperature

The temperature a parcel of mantle material would attain after a reversible adiabatic process bringing it to a specified reference pressure, without changing its composition or phase assemblage. It removes the compressional contribution when comparing sources at different depths. Within one phase and with depth $z$ positive downward, the adiabatic process gives the gradient is $dT/dz=\alpha gT/c_p$, soAbsolute temperature is used inside this formula. Recovering a source $T_p$ from erupted magma also requires accounting for melting, latent heat, ascent and fractional crystallization; an eruption temperature is not automatically a mantle potential temperature.