Elasticity of subducted basaltic crust at the lower mantle pressures: Insights on the nature of deep mantle heterogeneity

The origin of the large low velocity structures observed in the lowermost mantle is still unclear. To make new insights into deep mantle heterogeneity, we performed ab initio modeling of the elasticity of representative petrology with the pyrolitic and basaltic compositions based on the elasticities of the following mineral phases: magnesium silicate perovskite (post-perovskite), ferropericlase, calcium silicate perovskite, stishovite (CaCl2), and calcium ferrite (titanate)-type MgAl2O4. Calculations elucidated that the basalt composite is ∼2% denser than the pyrolitic one through all of the pressures in the lower mantle. In addition, the shear (VS) and bulk sound (VΦ) velocities were found to be ∼2% slower and faster in the basalt composition in particular at ∼30 GPa and >120 GPa respectively, while both the pyrolitic and basaltic composites have almost the same compressional wave speed (VP). Thermochemically distinct piles which are thought to be related to accumulated hot and dense subducted crusts would therefore be expected to produce as much as a −5% to −6% low velocity anomaly in the S wave but only ∼−1% in the P wave. These suggest that the basaltic pile seems less compatible to the seismological signatures of the large low velocity structures, if accompanied by thermal anomalies reaching +1000 to +1500 K.