Seismic heterogeneities and their nature in the lower mantle

Abstract

Over the last 20 years, global seismology has made significant progress in mapping the deep interior of the Earth. Tomographic studies identified variations in lower-mantle chemistry and phase transitions with depths of observed seismic heterogeneities occupying the entire range of the lower mantle. Three major zones of seismic heterogeneities can be outlined. The upper (shallow) zone from 660–1300 km includes ~70% of all heterogeneities, observed almost equally near subduction zones and beneath the tectonic plates. The middle zone, from 1300 to 1900 km, includes ~20% of all heterogeneities, which are observed entirely near subduction zones. The lower zone, from 1900 km to the border of D″ layer at 2700 km includes only a small number of heterogeneities. The deepest seismic heterogeneities, identified within the central parts of the Eurasian and North American plates, are located at depths of ~2630 km and ~2400 km. No correlation between the observed seismic heterogeneities and major mineral phase transitions and spin crossover were identified. The seismic heterogeneities, most likely, reflect local and regional chemical variations within the lower mantle.