Equation of state of lower mantle (Al,Fe)-MgSiO3 perovskite

The compression behavior of various (Al,Fe)-bearing MgSiO3 perovskites was investigated by powder X-ray diffraction up to 70 GPa on the ID30 beamline of ESRF (Grenoble, France). Using diamond anvil cell coupled with CO2 laser-heating, we obtained a reliable equation of state up to typical lower mantle pressures. In contrast to Fe which essentially increases the room pressure unit cell volume (V0), the effect of Al is to increase the bulk modulus of silicate perovskite. This result contrasts with previous determinations performed at pressures below 10 GPa on samples synthesized in the multi-anvil press. Such a difference can be explained by a change in the substitution mechanism of Al in MgSiO3 with increasing pressure and temperature, in agreement with recent ab-initio calculations. Our results confirm that the Earthʹs lower mantle (Mg+Fe)/Si ratio is greater than unity, because of the high stiffness of silicate perovskite.