K. K. M. Lee $^{a}$, B. O'Neill, W. R. Panero $^{b}$, S-H. Shim $^{a}$, L. R. Benedetti $^{c}$ and R. Jeanloz $^{a}$
$^{a}$ Department of Earth and Planetary Science, University of California, Berkeley, 94720-4767, USA $^{b}$ Department of Geological Sciences, University of Michigan, Ann Arbor 48109, USA $^{c}$ CEA, Bruyeres-Le-Chatel, France 91680
High-resolution x-ray diffraction to 107 GPa on an undepleted natural peridotite -- of pyrolite composition and representative of the Earth's upper-mantle -- transforms to the assemblage $\sim$76 ($\pm$ 2)\% (Mg$_{0.88}$ Fe$^{2+}$$_{0.05}$ Fe$^{3+}$$_{0.01}$ Al$_{0.12}$ Si$_{0.94}$)O$_{3}$ orthorhombic perovskite by volume (at zero pressure), 17 ($\pm$ 2)\% (Mg$_{0.80}$ Fe$_{0.20}$)O magnesiow\"{u}stite and 7 ($\pm$ 1)\% CaSiO$_{3}$ perovskite at lower-mantle conditions. The room-temperature bulk properties ($\rho$$_{0}$ = 4.13 ($\pm$ 0.02) g/cm$^{3}$, isothermal bulk modulus K$_{0T}$ = 247 ($\pm$ 21) GPa and pressure derivative K$_{0T}$' = 4.01 ($\pm$ 0.67)) of this high-pressure assemblage, together with a range of estimates of thermal properties of the constituent minerals, lead to quantitative predictions of density ($\pm$ 0.5\% uncertainty) and bulk modulus ($\pm$ 3\% uncertainty) for the high-pressure assemblage at lower-mantle pressures and temperatures. The values that we obtain are inconsistent with seismological constraints on the density, bulk modulus and temperature of the lower mantle. Therefore, we propose a lower mantle differing in composition (e.g., richer in iron) from current estimates for the upper mantle, implying some amount of segregation between the upper and lower mantle.