Weakening of calcium iridate during its transformation from perovskite to post-perovskite

The lowermost part of the Earths mantlethe ∼200-km-thick D′′ layershows anomalous seismic properties, and is rheologically distinct from the rest of the lower mantle 3,4 . The difference is thought to result from a phase transition from silicate perovskite to silicate post-perovskite. However, the rheology of the latter phase remains to be documented owing to experimental difficulties in reproducing pressures equivalent to those in the lowermost mantle. Here we address this problem by conducting laboratory experiments that use calcium iridate, which has been shown to be an appropriate low-pressure analogue. We find that the post-perovskite phase of this analogue is approximately five times weaker than its perovskite phase, and that it further weakens by a factor of two during the phase transformation; these are minimum estimates. If, as is likely, a similar weakening occurs in lower-mantle magnesium-silicate compositions, this could provide an explanation for the behaviour of the lowermost mantle as inferred from geophysical data.