Iron-rich perovskite in the Earth's lower mantle

The equations of state of perovskite with (Mg(0.75),Fe(0.25))SiO(3) and MgSiO(3) compositions have been investigated by synchrotron X-ray diffraction up to 130 GPa at 300 K in diamond anvil cells. Here we show that the addition of 25% Fe in MgSiO(3) perovskite increases its density and bulk sound velocity (V(Phi)) by 4-6% and 6-7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Mossbauer spectroscopic studies of the samples, the increase in V(Phi) and density can be explained by the occurrence of the low-spin Fe(3+) and the extremely high-quadrupole component of Fe(2+). Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of V(Phi) that is compatible with seismic observations of reduced shear-wave velocity in regions interpreted as dense, stiff piles in the lower mantle. Therefore, the existence of the Fe-rich perovskite in the lower mantle may help elucidate the cause of the lower-mantle large low-shear-velocity provinces (LLSVPs) where enhanced density and V(Phi) are seismically observed to anti-correlate with the reduced shear wave velocity. (C) 2011 Elsevier B.V. All rights reserved.