First Direct Shock Wave Loading Experiments on Wadsleyite

We are conducting the first direct shock wave loading experiments on Mg₂SiO₄ wadsleyite, one of the high-pressure polymorphs of olivine and a dominant phase in the Earth's mantle transition zone. Equation of State (EoS) measurements are performed simultaneously on two polycrystalline samples: 1) a disk of pure forsterite, hot-pressed to full crystal density (3.2-3.22 g/cc) at 3 GPa and 1573 K in the piston-cylinder; and 2) wadsleyite (3.37-3.4 g/cc), synthesized from forsterite at 16 GPa and 1373 K in a multi-anvil press using a special assembly designed to yield samples with large enough diameter for the EoS experiments. Our first experiment was conducted at a projectile velocity of 1.96 km/s on the Caltech 40-mm powder gun, using a tungsten flyer-driver combination. We used the inclined-mirror technique in order to constrain the behavior of both the elastic and deformational (plastic) shock waves in each sample. A two-wave structure was observed in the streak record of the forsterite, consistent with previous results. In the wadsleyite, however, the plastic shock wave superceded the elastic wave. The final shock states in the samples were: 1) forsterite, Up = 1.52 km/s; Us = 7.97 km/s; P = 38.8 GPa; ρ = 3.97 g/cc; 2) wadsleyite, Up = 1.51 km/s; Us = 7.82 km/s; P = 39.8 GPa; ρ = 4.18 g/cc. The result for forsterite is in accord with the previously established Hugoniot and with the EoS at 300K extrapolated from static compression experiments. The Hugoniot point for wadsleyite, however, gives a significantly higher density than that predicted (4.08 g/cc) for the same pressure along the 300 K isotherm (using K = 172 GPa, K' = 4.2). This suggests that, during shock compression, wadsleyite may partially convert to a higher-pressure phase or phase assemblage at conditions under which forsterite remains metastably in the low-pressure phase state. Obviously, more experiments are needed to confirm this trend. Future experiments are planned for both lower shock pressures on the 40-mm gun and higher shock pressures using the two-stage light-gas gun. By comparing the Hugoniots of the two samples, which are shocked to different internal energy states at equal volume, we expect to gain constraints on the Gruneisen parameters of the polymorphs as well as their breakdown products at higher pressures (periclase and MgSiO₃-perovskite).