Stochastic tomography, made possible by dense deployments of seismic sensors, is used to identify previously undetected and poorly detected changes in the composition and mineral structure of Earth’s mantle. This technique inverts the spatial coherence of amplitudes and travel times of body waves to determine the depth and lateral dependence of the spatial spectrum of seismic velocity. The inverted spectrum is interpreted using predictions from the thermodynamic stability of different compositions and mineral phases as a function of temperature and pressure, in which the metamorphic temperature derivative of seismic velocities can be used as a proxy for the effects of heterogeneity induced in a region undergoing a phase change. Peaks in the metamorphic derivative of seismic velocity are found to closely match those found from applying stochastic tomography to elements of Earthscope and FLEX arrays. Within ± 12 km, peaks in the fluctuation of P velocity at 425, 500, and 600 km depth beneath the western US agree those predicted by a mechanical mixture of harzburgite and basalt in a cooler mantle transition zone. A smaller peak at 250 km depth may be associated with chemical heterogeneity induced by dehydration of subducted oceanic sediments, and a peak at 775 km depth with a phase change in subducted basalt. Non-detection of a predicted endothermic phase change near 660 km is consistent with its width being much less than 10 km. These interpretations of the heterogeneity spectrum are consistent with the known history of plate subduction beneath North America.

Peaks are observed in a depth dependent power spectrum of P-wave velocity fluctuations determined from an inversion of P wave coherences observed by the USArray. These peaks correlate with the depths of the majority of silicate mineral phase changes predicted by a thermodynamic model of the upper 1000 km of a pyrolitic mantle. To within ± 25 km we identify the phase change of orthopyroxene to HP-clinopyroxene at 275 km, the olivine to wadsleyite phase change at 425 km, the wadlsleyite to ringwoodite phase change at 505 km, and the initiation of an akimotoite phase at 600 km and a signature of a phase change at 775 km, both associated with the existence of fragments of subducted oceanic crust. Non-detection of a phase change at or near 660 is consistent with the phase change of ringwoodite to Mg-perovskite and magnesiowusite occurring over a depth interval much smaller than 25 km.