The elastic properties and anisotropic behavior of MgSiO3 akimotoite at transition zone pressures

Abstract Seismic wave velocities in the Earth's transition zone, between 410 and 660 km depth, are poorly matched by mineralogical models. Mainly due to the presence of majoritic garnet, wave velocities calculated for a peridotite composition lithology are slower than seismic reference models, particularly towards the base of the transition zone. One possible resolution for this discrepancy is if the MgSiO3 polymorph akimotoite replaces majoritic garnet in some regions of the transition zone. This is possible in peridotitic material if temperatures are slightly lower than a typical geotherm or in harzburgitic composition material. The presence of akimotoite might serve as an explanation not only for the discrepancy of seismic velocities at the base of the transition zone but also for observations of transition zone seismic anisotropy in regions of current subduction. In order to provide the data required to test this possibility, two high-quality single-crystals of MgSiO3 akimotoite were studied using combined Brillouin spectroscopy and X-ray diffraction up to 24.86(3) GPa, i.e. to the limit of the akimotoite stability field. The resulting equation of state yields an adiabatic bulk modulus and first pressure derivative of KS0 = 209(2) GPa and K′ = 4.4(1), respectively, and a shear modulus and first pressure derivative of G0 = 131(1) GPa and G′ = 1.7(1). This is in overall agreement with several experimental and computational studies, however, the resulting aggregate velocities are slower than previously reported, especially at high pressures. The elastic anisotropy of akimotoite is found to decrease only slightly with pressure; akimotoite hence remains the most elastically anisotropic mineral in the transition zone and may therefore play a major role in explaining seismic anisotropy observations in the proximity of subducting slabs.