Equations of state of the high-pressure phases of a natural peridotite and implications for the Earthʹs lower mantle

To help determine the chemical composition of the Earthʹs mantle, we characterised the high-pressure mineral assemblage of an undepleted natural peridotite—thought to be representative of the Earthʹs upper-mantle—to 107 GPa using high-resolution X-ray diffraction. At lower-mantle conditions, the peridotite transforms to the assemblage 76 (±2)% (Mg0.88Fe0.052+Fe0.013+Al0.12Si0.94)O3 orthorhombic perovskite by volume (at zero pressure), 17 (±2)% (Mg0.80Fe0.20)O magnesiowüstite and 7 (±1)% CaSiO3 perovskite. The measured room-temperature bulk properties of this high-pressure assemblage, together with a range of estimates of thermal properties of the constituent minerals, appear to be inconsistent with seismological constraints on the density and bulk modulus of the lower mantle. Our results suggest that the lower mantle differs in bulk composition (e.g., richer in iron, Mg#∼0.85) from current estimates for the upper mantle, requiring some amount of segregation between the upper and lower mantle over geological history.