The effect of Fe and Al substitution on the compressibility of MgSiO3-perovskite determined through single-crystal X-ray diffraction

A compressibility study on three well characterized single crystals of magnesium silicate perovskite containing variable proportions of iron and aluminium has been carried out using X-ray diffraction in a diamond anvil cell in order to access the effect of varying chemical composition on perovskite elastic properties. The single crystals were synthesized using a multianvil apparatus at 25 GPa and 1800–2000 °C. It was found that the substitution of Fe2+ for Mg2+ in Al-bearing perovskite increases the ambient pressure unit-cell volume to a larger extent than when the same substitution occurs in Al-free perovskite. Equation of state measurements were performed up to pressures of 9 GPa at room temperature. With increasing Fe and Al incorporation perovskite exhibits a modest increase in compressibility compared to MgSiO3-perovskite over the pressure range investigated. A 3rd order Birch–Murnaghan equation of state fit to the compression data indicates that the bulk modulus (K0), decreases from 243 to 234 GPa while K′ increases from 5.0 to 6.3 as the Fe and Al contents increase from approximately 5 to 17 and from 2 to 8 wt.%, respectively. The high values of K′ will result in Fe–Al-bearing perovskites becoming less compressible than MgSiO3 perovskite at pressures >10 GPa. Extrapolating these results to lower mantle conditions indicates a pronounced effect of composition on density in the upper portions of lower mantle, with perovskites formed in subducted oceanic crust, i.e. with higher Fe and Al contents, being substantially denser than those formed within a primitive mantle composition. This density contrast diminishes, however, with increasing pressure, which also contributes to a stronger dependence of bulk sound velocity on composition towards the base of the lower mantle.