Iron-rich perovskite in the Earth's lower mantle

The equations of state of perovskite with (Mg0.75,Fe0.25)SiO3 and MgSiO3 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 MgSiO3 perovskite increases its density and bulk sound velocity (VФ) 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 VФ and density can be explained by the occurrence of the low-spin Fe3+ and the extremely high-quadrupole component of Fe2+. Combining these experimental results with thermodynamic modeling, our results indicate that iron-rich perovskite can produce an increase in density and a value of VФ 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 VФ are seismically observed to anti-correlate with the reduced shear wave velocity.

► We have studied the equation of states of perovskite with 25% Fe and without Fe. ► A 4-7% increase in the density and VΦ at lower-mantle pressures was observed. ► The increase is explained as a result of the low-spin Fe3+ and high QS Fe2+. ► 4.An excess of 15% iron in perovskite may explain large-low-shear velocity provinces.