Dynamic compression of iron oxides to multi-megabar pressures

Laser-driven dynamic compression enables the study of planetary materials at the extreme pressure-temperature (PT) conditions of Earth’s deep interior. When coupled with in situ x-ray diffraction, fundamental properties such as crystal structure and equation of state can be accurately measured which are essential for developing interior structure, dynamics, and evolution models for planets within and outside our solar system. The discovery of over 5500 extrasolar planets has motivated research on matter at even more extreme PT conditions as calculations based on density estimates for these planets suggest PT profiles that greatly exceed those for Earth. As planetary detection techniques and mass-radius measurement precision continue to improve, experimental efforts must accompany these advances to better understand the interiors of these planets which largely control their surface processes, including planet habitability.

In this seminar, I will present results from two such studies where in situ x-ray diffraction is used to diagnose the phase stability and crystal structures of Fe₂O₃ and Fe₃O₄ dynamically compressed to ~700 GPa. Iron oxides have recently been shown to act as important analogs of Mg-silicates (i.e., MgSiO₃ and Mg₂SiO₄), and as the most abundant multivalent element in rocky composition planets, important markers of mantle redox. On the nanosecond time scales of our experiments, we observe that hematite-type Fe₂O₃ transforms to the theta-Fe₂O₃ phase, predicted to be metastable under static loading conditions. Above ~350 GPa, no sample diffraction peaks are observed suggestive of pressure-induced amorphization. In contrast, we observe diffraction consistent with the crystalline Th₃P₄-type Fe₃O₄ above ~250 GPa and up to the highest achieved pressure. Our results provide important insights into exoplanet mineralogy and, when combined with previous works on Fe and FeO, are used to develop a compositional model for the Earth’s solid inner core.