The investigators are carrying out a three-year program to determine the elastic properties of a wide range of Earth materials at high pressures (P) and temperatures (T) by Brillouin scattering measurements of sound velocities. Their goal is to measure the velocities of the most important materials comprising the transition zone of the Earth (from 410-660 km depth), the lower mantle, subducting tectonic plates and the adjacent upper mantle. The motivation for this work stems from seismological studies of Earth?s deep interior, which provide the velocities of longitudinal (VP) and shear (Vs) sound waves as a function of depth, and the lateral variations of velocity relative to global average models. These seismic results are the most direct information on the in-situ properties of Earth?s mantle, thus providing strong constraints on chemical, mineralogical, thermal, and geodynamic models of the deep interior, and evolution of the Earth through time. The sound velocity measurements are critical for fully utilizing the seismic data, and they are making these measurements under high pressure and high temperature conditions that closely mimic the actual conditions in the Earth at great depth.
The investigators are pursuing two broad classes of activities: 1) Measurements of velocities and elastic properties at high pressures up to the 100 GPa region (at room T), and high temperature measurements at normal pressure. The first priority is measurements on what are believed to be the most abundant solid phases in the Earth?s mantle (to 2900 km depth). They are attempting to measure how changes in the electronic state of iron in these minerals affect on the sound velocities, and are performing some preliminary measurements on hydrous phases that carry hydrogen (or water) into the deep earth in subducting tectonic plates and store it there. 2) The team is continuing to develop an apparatus to measure the sound velocities of minerals at the actual pressure-temperature conditions of the mantle. This is being done by Brillouin measurements on samples compressed in a diamond anvil cell (DAC) and heated with an infrared laser. They expect to measure velocities on minerals at conditions of at least T=2500 K and P>50 GPa, spanning the likely range of P-T conditions that exist in the Earth at depths of about 1300 km. Lower-temperature measurements are being made with an electrically-heated DAC.
The results of these experiments will provide far stronger constraints than are currently available on the chemical composition and temperature of the Earth?s interior. The community will be positioned to more fully exploit the potential of detailed seismic information for understanding the structure of the Earth and its heterogeneity at depth. These experiments are also pushing forward the technology of experimental velocity measurements at elevated P-T conditions, into a regime that has previously been the realm of theory alone. The expected findings are of benefit to the entire spectroscopy community (chemistry, physics, materials sciences, etc.) interested in experiments under extreme conditions. This research involves the training of graduate and undergraduate students. These students have a unique opportunity to be involved in cutting-edge experimental research and be trained in some of the most advanced experimental techniques in this field.
