(Fe, Al)-bearing post-perovskite in the Earth’s lower mantle
Thursday, 18 December 2014
In the past two decades, a number of seismic velocity anomalies have been identified in the bottom 200-300 km of the lower mantle above the core-mantle boundary (CMB). Deciphering the observed seismic anomalies in the region has attracted extensive research interests. Of particular importance is the discovery of Mg-silicate perovskite (Pv) to post-perovskite (PPv) phase transition, which has shed light on understanding the abnormal behavior above the CMB. In this study, we have investigated the combined effect of Fe and Al on the electronic spin and valence states as well as the equation of state of PPv using synchrotron X-ray diffraction and Mössbauer spectroscopy in high-pressure diamond anvil cells. We have synthesized two PPv samples, Mg0.6Fe0.15Al0.5Si0.75O3 and Mg0.66Fe0.13Al0.28Si0.86O3, at ~165 GPa and 2200-2500 K. By analyzing the collected Mössbauer spectra, we found that most Fe2+ and Fe3+ occupy the large bipolar prismatic sites in both of our samples. All of the Fe remains in the high-spin state at ~165-168 GPa and 300 K, which is consistent with theoretical predications. Together with the X-ray diffraction results, we have noted that the unit cell volume and density of PPv are significantly affected by the spin and valence states of Fe and the Al substitution. Using these results, we have further modeled the density and velocity change across the perovskite and PPv phase transition. The combined effect of Fe and Al will cause an increase in density but a decrease in the bulk sound velocity. As a result, the Fe and Al rich PPv existing above the CMB would be shown as relatively high-density and low-velocity regions.