Inhomogeneity parameter of a homogeneous Earth

Abstract

Adiabatic and chemically homogeneous solid regions of the Earth are expected to appear otherwise because of the bulk attenuation to seismic waves that occurs in polyphase aggregates such as the Earth's mantle. Because the seismologically measured bulk modulus is only partially relaxed compared with the hydrostatic pressure–density relation (or relaxed modulus) of the Earth's mantle, an apparently nonadiabatic contribution1 to the geotherm is expected to be inferred from seismological observations on regions of the mantle that are well mixed by vigorous convection. Using a recent analysis of bulk attenuation in polyphase assemblages2, we estimate the total nonadiabatic contribution to the geotherm through the lower mantle to be approximately −300 to −800 K. The magnitude of this effect is significant in that it is of the same order as the temperature change within a thermal boundary layer, which represents the largest perturbation of the geotherm resulting from convection within the Earth. Moreover, if the lower mantle is truly homogeneous and adiabatic, it should appear to be subadiabatic according to the seismological data.