Elasticity of (Mg_(0.83),Fe_(0.17))O ferropericlase at high pressure: ultrasonic measurements in conjunction with X-radiation techniques

摘要

The elasticity of ferropericlase with a potential mantle composition of (Mg_(0.83),Fe_(0.17))O is determined using ultrasonic interferometry in conjunction with in situ X-radiation techniques (X-ray diffraction and X-radiography) in a DIA-type cubic anvil high-pressure apparatus to pressures of 9 GPa (NaCl pressure scale) at room temperature. In this study, we demonstrate that it is possible to directly monitor the specimen length using an X-ray image technique and show that these lengths are consistent with those derived from X-ray diffraction data when no plastic deformation of the specimen occurs during the experiment. By combining the ultrasonic and X-ray diffraction data, the adiabatic elastic bulk (K_S) and shear (G) moduli and specimen volume can be measured simultaneously. This enables pressure scale-free measurements of the equation of state of the specimen using a parameterization such as the Birch-Murnaghan equation of state. The elastic moduli determined for (Mg_(0.83),Fe_(0.17)O are K_(S0) = 165.5(12) GPa, G_0 = 112.4(4) GPa, and their pressure derivatives are K_(S0)'=4.17(20) and G_0'=1.89(6). If these results are compared with those for MgO, they demonstrate that K_(S0) and K_(S0)' are insensitive to the addition of 17 mol% FeO, but G_0 and G_0' are reduced by 14% and 24%, respectively. We calculated that the P and S wave velocities of a perovskite plus ferropericlase phase assemblage with a pyrolite composition at the top of the lower mantle (660 km depth) are lowered by 0.8 and 2.3%, respectively, when compared with those calculated using the elastic properties of end-member MGO. Consequently, the magnitudes of the calculated wave velocity jumps across the 660 km discontinuity are reduced by about 11% for P wave and 20% for S wave, if this discontinuity is considered as a phase transformation boundary only (ringwoodite→perovskite+ferropericlase).