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The mineralogy of the Earth’s upper mantle is well-understood through direct geochemical and petrological observations, indicating its pyrolytic chemical composition with a Mg/Si molar ratio of approximately 1.3. In contrast, the mineralogy of the lower mantle, constituting over 50% of Earth's volume and mass, remains a subject of intense debate, which questions whether the lower mantle is chemically homogeneous with the upper mantle through whole-mantle mixing via lithospheric slab subduction into the lowermost mantle, or if it comprises a chemically distinct layer enriched in silica or bdm from the upper mantle. This issue is an essential challenge in understanding the evolution of Earth's interior. One of the most promising approaches to place a robust constraint on this issue has been considered to experimentally measure the sound wave velocities of constituent mineral phases of the lower mantle, which consists mainly of (Fe, Al)-bearing MgSiO3 bdm, (Mg, Fe)O ferropericlase (fp) with a small amount of CaSiO3-perovskite (CaPv), covering entire range of lower mantle pressure conditions and directly compare them with the 1-D seismological model such as PREM.
In order to address this issue, I would discuss about the in-situ ultrahigh-pressure transverse sound velocity (VS) measurements for polycrystalline Fe3+-rich (Fe, Al)-bearing bdm at pressures up to 115 GPa, effectively spanning the entire range of lower mantle pressures using a Brillouin scattering spectroscopic technique in a diamond anvil cell apparatus (DAC) in conjunction with Raman scattering measurements. From the experimentally derived shear coefficients, I will focus on the modelling for the lower mantle mineralogy. |