Effect of chemistry on the compressibility of silicate perovskite in the lower mantle

摘要

Three single-crystals of magnesium silicate perovskite with differing chemical compositions have been studied by means of synchrotron X-ray diffraction in diamond anvil cells with He as pressure transmitting medium from room pressure up to 75GPa. In addition to the end-member MgSiO _3 composition, a perovskite containing 4mol% of the Fe ~(2+)SiO _3 component [(Mg,Fe)SiO _3] and one containing 37mol% of an Fe ~(3+)AlO _3 component [(Mg,Fe)(Al,Si)SiO _3] were investigated. The high-quality of the collected data allows a detailed examination of the effect of different chemical substitutions on the compression mechanism of perovskite and on its equation of state (EoS). The bulk modulus and first pressure derivative determined for MgSiO _3 perovskite obtained by fitting a 3rd-order Birch-Murnaghan are found to be quite insensitive to the maximum pressure to which the data are fitted. The EoS parameters obtained by fitting data from room pressure to 10, 40 or 75GPa are almost identical. This is not the case, however, for either (Mg,Fe)SiO _3 or (Mg,Fe)(Al,Si)SiO _3 perovskites, for which volumes calculated from an EoS obtained from fitting data up to 40GPa deviate from the experimental data above 40GPa. In the case of (Mg,Fe)SiO _3 perovskite this deviation appears to be related to a change in octahedral tilting during compression, as revealed by analysis of the lattice strain variation with pressure. The tilting change is a likely consequence of a high-spin to intermediate spin transition of Fe ~(2+) but the effect on the density and bulk modulus is almost negligible and unlikely to cause seismically observable changes in the mantle. In the case of (Mg,Fe)(Al,Si)SiO _3 perovskite, the deviation is clearly due to a change in the compressibility of the c-axis and no evidence for effects due to a change in Fe spin state is observed. Substitution of Fe ~(2+)SiO _3 has a significant negative effect on the bulk sound velocity while increasing density, whereas the effect of Fe ~(3+)AlO _3 substitution on both the bulk sound velocity and density are in the same direction but more modest. The latter substitution may, therefore, be more compatible with some aspects of seismic anomalies observed at the base of the lower mantle.