Weak cubic CaSiO 3 perovskite in the Earth's mantle.

Cubic CaSiO 3 perovskite is a major phase in subducted oceanic crust, where it forms at a depth of about 550 kilometres from majoritic garnet 1,2,28 . However, its rheological properties at temperatures and pressures typical of the lower mantle are poorly known. Here we measured the plastic strength of cubic CaSiO 3 perovskite at pressure and temperature conditions typical for a subducting slab up to a depth of about 1,200 kilometres. In contrast to tetragonal CaSiO 3 , previously investigated at room temperature 3,4 , we find that cubic CaSiO 3 perovskite is a comparably weak phase at the temperatures of the lower mantle. We find that its strength and viscosity are substantially lower than that of bridgmanite and ferropericlase, possibly making cubic CaSiO 3 perovskite the weakest lower-mantle phase. Our findings suggest that cubic CaSiO 3 perovskite governs the dynamics of subducting slabs. Weak CaSiO 3 perovskite further provides a mechanism to separate subducted oceanic crust from the underlying mantle. Depending on the depth of the separation, basaltic crust could accumulate at the boundary between the upper and lower mantle, where cubic CaSiO 3 perovskite may contribute to the seismically observed regions of low shear-wave velocities in the uppermost lower mantle 5,6 , or sink to the core-mantle boundary and explain the seismic anomalies associated with large low-shear-velocity provinces beneath Africa and the Pacific 7-9 .