Identifying dehydration-induced shear velocity anomaly in the Earth's core-mantle boundary

Public summary

* The chemistry of hydrous minerals is investigated at the conditions of the lowermost mantle.

* Seismic waves propagate faster in regions where mineral dehydration occurs.

* The incorporation of 3.5 wt % water explains the enigmatical increase of seismic velocity at the bottom of the Farallon slab.

* Water-mineral chemistry shapes the structure of Earth’s lowermost mantle.

Abstract

The steep temperature gradient near the bottom of the mantle is known to generate a negative correlation between the shear wave velocity (VS) and the depth in most regions of the D″ layer, as detected by seismological observations. However, increasing VS with depth is observed at the D″ layer beneath Central America, where the Farallon slab sinks, and the origin of this anomaly has not been well constrained. Here, we calculate the thermoelastic constants and obtain the elastic wave velocities of hydrous phase H with various Al contents and cation configurations, which may act as a water carrier to the D″ layer. We find its VS to be substantially lower than the post-perovskite-type bridgmanite. The dehydration of Al-enriched phase H and the redistribution of Al from the hydrous component to dry silicates would gradually raise the VS below the top of the D″ layer. The presence of 3.5 wt % water is sufficient to compensate for the thermal effects to match the seismic anomaly at the bottom of the mantle beneath Central America. The positive slope of VS versus depth in the D″ layer may fingerprint deep water recycling.