Ultrasonic Velocity Measurements on Multiphase Aggregates and Constituent Minerals of the Mantle at High Pressure and High Temperature

The chemical composition of the Earth's deep mantle holds important information for many processes at the surface, including the global carbon budget and water cycles, earthquakes, and plate tectonics. Since no samples from depths greater than 12 km are available for direct laboratory investigations, the team's approach is to bring candidate rock and mineral specimens back to the deep mantle pressure and temperature conditions inside a laboratory apparatus, and then conduct in-situ experiments to understand the physical and chemical states and properties of these minerals at mantle depths. In comparison with previous studies, this project marks a major advancement by measuring directly the sound velocities in synthetic rock samples which bear a closer resemblance to those in the real Earth. From both technological and scientific aspects, these technological advances can be readily applied to the study of non-geoscience substances, such as those important to industrial and energy sciences and for the designing and discovery of new materials. The PIs propose to conduct direct measurements of compressional [P] and shear [S] wave velocities and densities on multi-phase aggregates of pyrolitic composition and mantle constituent minerals (KLB-1 pyroxenes; coesite and stishovite) up to the P-T conditions of the top of the lower mantle using ultrasonic interferometry in conjunction with synchrotron X-radiation. These directly measured velocities of aggregates take into account the effects of chemical reactions among co-existing phases (phase fractions, minor element partitioning) in the equilibrated specimens as well as the effect of minor elements on elasticity and can be compared directly with seismic velocity-depth profiles. In addition to yielding phase equilibria for pyrolitic compositional model as a function of depth, these unprecedented results will provide critical data for the interpretation of seismically-derived velocity and density vs. depth profiles and place constraints on the mineralogical composition and dynamic processes of the Earth's interior.