Seismological signature of chemical differentiation of Earth's upper mantle

Chemical differentiation from pyrolite to harzburgite due to partial melting and melt extraction process causes the chemical heterogeneity in Earth's upper mantle that can be detected by seismological observations. The variation in major element chemistry in natural samples reflects complicated processes that include not only partial melting but also other various magmatic processes. On the basis of a comparison of chemical and mineralogical compositions of natural peridotites with those from melting experiment, density and seismic velocities of various peridotites are calculated for the range of pressure and temperature in the upper mantle using the latest data on mineral thermoelasticity. We conclude that the seismic velocities of shallow oceanic peridotites is characterized by a single parameter such as Mg # (molar ratio Mg/(Mg + Fe)), whereas the characterization of the deep continental peridotites requires two parameters, Mg # and Opx # (volume fraction of orthopyroxene). In agreement with previous studies, we find that in spinel stability field, the seismic velocities have positive correlation with Mg # from pyrolite to residual harzburgite, while in garnet stability field, seismic velocities of residual harzburgite are indistinguishable from those of pyrolite. The seismic velocities of the deep continental peridotites are lower than those of pyrolite and residual harzburgite because of the high concentration of orthopyroxene with low seismic velocities and have large pressure dependence. A jump of seismic velocity will occur at 300 km in orthopyroxene‐rich continental harzburgite due to the orthorhombic to high‐pressure monoclinic phase transition in (Mg, Fe)SiO 3 pyroxene. This phase transition may correspond to the X discontinuity.