Iron-rich perovskite in the Earth's lower mantle

Abstract

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 Ф) by 4–6% and 6–7%, respectively, at lower-mantle pressures. Based on concurrent synchrotron X-ray emission and Mössbauer spectroscopic studies of the samples, the increase in V Ф 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 Ф 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 Ф are seismically observed to anti-correlate with the reduced shear wave velocity.