The process of irreversible adiabatic decompression and melting of mantle rocks has been evaluated using the thermodynamic condition of frictionless decompression in a gravitational field. It is found that the melt productivity is significantly greater, thus enhancing the tendency for melt segregation, if a parcel of mantle rock moves upward at nearly constant velocity than that derived for the case of isentropic decompression. The T-Z trajectory of a solid mantle diapir moving upward at a constant velocity has, in general, a smaller slope than the isentropic trajectory, and assumes a negative slope when its density falls below ~94% of that of the surrounding mantle. The acceleration of upward movement, and the overall density reduction of the material due to phase change that is the likely cause for the acceleration, have qualitatively opposite effects on the deviations of melt productivity and T-Z trajectory of mantle diapir from those under isentropic conditions.