Located nearly 3000 km below the crust, the core represents the most remote region within our planet. Seismology and geodynamics have provided detailed descriptions of the core and its central role in the Earth’s evolution and its dynamic processes. Determining the high pressure-temperature behavior of Earth materials is an important but challenging area of research which is integral for gaining insight into our planet’s deep interior.  We conducted experiments which simulate the high pressures and temperatures that exist with the deep Earth and measured how the properties of planetary materials can be dramatically altered at extreme conditions.  In addition, we have made technical advances which will be useful to other experimentalists in the geosciences as well as fundamental and applied sciences. We disseminated our results in a timely manner through presentations at national and international meetings, conferences, workshops, and publications.  We also actively publicized our work by partnering with the communications and press offices at Stanford and SLAC. The success of this project has depended on the participation of a number of young, female researchers whose training and career development was supported. In addition a number of high school and undergraduate summer interns were also supported by this project.

We used a two-pronged experimental approach, specifically by conducting both static and dynamic compression experiments. The first direction uses a static pressure device called a diamond anvil cell which is coupled to x-ray techniques to study the materials in-situ at high pressures and temperatures at high pressures and temperatures.  The second approach is focused on reaching core conditions via laser-driven shock experiments and developing diagnostics to characterize the samples. The combination of the static and dynamic results will provide important information for helping to understand and interpret the complex seismic signatures in the Earth’s core at the boundary where core meets the overlying rocky mantle.  Some highlights from the static side include that we investigated the effect of nickel of the strength of Fe. We also looked at the effect of water and carbon-bearing phases for improving our understanding of deep volatile cycles. We found that iron reacts with water at deep lower mantle conditions to form a new pyrite-structured iron oxide, FeO₂H_x.  This phase is extremely rich in oxygen and has physical properties consistent with ultralow velocity zones that are observed just above the core-mantle boundary. This discovery has opened other exciting avenues to explore. For example, we recently demonstrated that oxygen in this phase has a -1 charge (while iron is in its ferrous or +2 state). This discovery has many implications, including the possibility that oxygen could have multiple oxidation states in the lower mantle. If present, these phases could play a significant role in the cycling of hydrogen and oxygen in the deep mantle. We also investigated the high pressure-temperature behavior of carbonates and found that the carbon coordination increases from three to four at deep mantle conditions.

 On the dynamic side, we made considerable progress with the development of diagnostics to detect transverse shocks on the Janus laser. We have shown that by using a special sample design that we developed, the shear wave velocity of a material can be measured directly during shock compression. Our data show we can independently validate the assumed melt point of iron from earlier sound speed data and provide the first ever direct shear wave measurements of iron to core pressures.  We find evidence of shear wave softening preceeding melt which when compared to seismic observations and can help constrain the nature of the inner core composition by providing a baseline for pure iron shear wave speeds at relevant conditions. Adding another dimension to our dynamic compression studies – time-resolution – we have completed a number of studies exploring the phase transition kinetics of Earth-relevant materials. Each of these studies give insight into the rheological properties, behavior and transformation mechanisms of materials in the Earth's crust, mantle, down to the inner core or cores of super-Earth exoplanets. Understanding phase transitions requires novel measurements like the ones we have made here under during grant, and we are ushering in a new era in geophysics where atomic-level spatial fidelity and material-response timescales are accessible at relevant extreme conditions.

Last Modified: 03/27/2020
Modified by: Wendy Mao