High Pressure-Temperature Single-Crystal Elasticity of the Lower-Mantle Bridgmanite

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Earth's lower mantle extends from 670 km depth down to the core-mantle boundary, 2900 km deep. There, pressure and temperature exceed 1.3 million atm and 3500 K (5840 degree Fahrenheit). Understanding its properties is critical to constrain the planet dynamics. The lower mantle conducts heat away from the outer core. This contributes to power the Earth's magnetic field which shields us from the solar wind. Furthermore, thermal convection in the mantle drives plate tectonics and associated hazards, such as earthquakes and volcanic eruptions. Seismology, the study of seismic (elastic) waves, allows to observe directly the lower-mantle structures. But their interpretation requires knowledge of the elastic properties of the constitutive minerals. Bridgmanite accounts for more than 3/4th of the volume of the lower mantle, making it the most abundant mineral in the Earth. It is also of crucial interest when investigating lower-mantle properties. Here, the team quantifies experimentally the elastic properties of bridgmanite at the extreme conditions of the deep Earth. Coupling high pressure devices and state-of-the-art analytical techniques, they provide data allowing the interpretation of lower-mantle structures; notably that of large enigmatic provinces showing low seismic shear velocities. The project has strong implications in Seismology and broad impacts in Geodynamics. It also provides support and training in Mineral Physics for several graduate and undergraduate students, as well as educational outreach toward local elementary and middle schools. In this study, the team synthesize large single crystals of (Al,Fe)-bearing bridgmanite in the laboratory. Crystal properties are investigated at extreme conditions of pressure and temperature in the diamond-anvil cell. This apparatus generates high pressures at the tip of two opposing diamonds. The high temperatures are obtained by external heating or using focused laser beams. Crystal elastic properties are measured in situ using a combination of laboratory Brillouin and impulsive stimulated light scattering, as well as X-ray diffraction at national synchrotron facilities. This is achievable because the team is developing new technology in time-resolved impulsive laser spectroscopy. These new techniques will be shared with the community for future studies of material properties at extreme conditions. The researchers use the obtained data to constrain bridgmanite full elastic moduli as a function of pressure, temperature and of its contents in iron and aluminum. Extrapolation of the results to conditions relevant to the lower mantle, as well as modeling, allows interpreting seismological observations. The project outcomes further the understanding of seismic velocities, temperature profiles and chemical compositions in the deep Earth.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.