부산대학교 도서관

Seismic anisotropy in the Earth's lower mantle likely results from a combination of elastic anisotropy and lattice preferred orientations of its main constituent minerals. As the second most abundant component of the lower mantle, ferropericlase has been widely studied, and the experimental results demonstrated, in general, a growing with pressure elastic anisotropy up to 1 Mbar. However, the unique measurements on the endmember (MgO) at comparable pressure conditions contradict the above observations and theoretical results. Here, time‐domain Brillouin scattering was applied to measure longitudinal sound velocities in single crystals of MgO compressed in diamond anvil cell. Velocities along two specific crystallographic directions, [100] and [111], were independently collected to 43 GPa. Applying the known bulk modulus, a complete set of single‐crystal elastic moduli, elastic anisotropy and aggregate shear modulus were derived. Our results revealed a steadily increasing with pressure elastic anisotropy at P > 20 GPa, consistent with the previous theoretical predictions and measurements on ferropericlase with moderate amounts of iron. Plain Language Summary: Variations in seismic wave velocities in the Earth's deep mantle can be explained, at least partially, by preferred orientations (caused by the mantle convection) combined with elastic anisotropy of one of the main constituents of the lower mantle, namely ferropericlase, (Mg1‐x,Fex)O. However, experimental and theoretical information on pressure dependence of its elastic anisotropy deviates from that measured for the end‐member MgO even though the predictions for MgO agree with those for (Mg1‐x,Fex)O. This is because measurement of elastic response of single crystals to uniaxial stress, for example, single‐crystal elastic moduli (Cij) and, accordingly, of elastic anisotropy at ultrahigh pressures is still challenging. Here, we measured Cij of MgO to the same degree of compression as in the upper part of the Earth's lower mantle using the advanced all‐optical technique adapted to diamond anvil cell which permits generation of ultrahigh pressures. We recorded an inversion of the elastic anisotropy of MgO upon compression and determined pressure dependence of its aggregate shear modulus. These results agree with the previously published data on ferropericlase. The influence of iron content on the elastic properties of (Mg1‐x,Fex)O at high pressures was discussed. Key Points: Sound velocities along specific directions in MgO single crystals were measured to 43 GPa using time‐domain Brillouin scatteringUpon compression, the Zener anisotropy ratio of MgO decreases steadily from above unity to below unityThe aggregate shear moduli of MgO and (Mg1‐x,Fex)O increase with pressure similarly but that of (Mg1‐x,Fex)O is smaller [ABSTRACT FROM AUTHOR]