Constraints on the presence or absence of post-perovskite in the lowermost mantle from long-period seismology

Abstract

In this study, the ability to detect post-perovskite is examined using three long-period seismology approaches; normal modes, arrival time variations, and seismic tomography. Although normal modes provide robust integral constraints on 1D velocity and density profiles of the Earth, their ability to resolve variations in shear velocity and density decreases near the core-mantle boundary (CMB). Therefore, it is possible for post-perovskite to exist globally within 200 km of the CMB without resolution by normal modes. Deep-turning long-period S wave arrival times are examined in regions of dense ray coverage and a shift representing an increase in velocity consistent with presence of post-perovskite is observed in localized areas approximately 100 km above the CMB. Therefore, post-perovskite is not a ubiquitous feature but may be locally stable within both tomographically slow and fast regions near the CMB. To globally determine if regions are cold or iron-enriched enough for post-perovskite to be stable, recent thermo-chemical models are applied to a suite of geotherms and phase transition depths. The results suggest that variations in iron content are too small to control the stability of post-perovskite. It is demonstrated that only a narrow range of temperatures and phase transition depths can allow post-perovskite to exist in anomalously fast (cold) regions in the lowermost mantle. Furthermore, if post-perovskite is a major constituent of fast regions, then the temperature at 2780 km depth ranges from 2400 K - 2700 K, and post-peorvskite does not explain the observed anti-correlation of shear velocity and bulk sound speed near the CMB.