Effects of phase transions and possible compositional changes on the seismological structure near 650 km depth

Abstract

If the lower mantle is enriched in Fe ± Si relative to the upper mantle, the compositional boundary is expected to be an interface with a density jump of ∼5 (±2)% and with depth varying laterally by >100–300 km, but with changes in seismic velocities that are nearly unobservable: ∼0 to −1.0 (± 2.5)% discontinuities in compressional (VP) and shear (VS) wave velocities. In contrast, the polymorphic transformation of transition‐zone minerals to the perovskite‐bearing assemblage of the lower mantle is estimated to produce a jump of ∼5–10 percent in density, VP and VS near 650 km depth, and the depth of the seismological discontinuity due to the phase transition is relatively independent (<50–150 km variation) of temperature or horizontal position. If present, a compositional boundary is not in general expected to occur at the identical depth (∼650 km) as the phase transition to the perovskitebearing assemblage, although the depths of the two may be linked.