Session Z05: Minerals under dynamic compression

Untrafast solid-state phase transformation of silicate minerals during shock compression

Meteorites keep unique evidence of their evolution histories in the solar system. Upon impacts of their parent asteroids, meteorites had recorded such evidences within a series of crystal structures of their dense silicate minerals, which emerged after phase transformations triggered by shock compression. Olivine [α-(Mg,Fe)₂SiO₄] and its three high-pressure phases, as discovered in such shock-experienced meteorites [1,2], are of particular interest. It is because actual conditions of the ancient shock compression processes were potentially recorded in a quantitative manner within these phases.

In this study, we try to experimentally evaluate the actual time and pressure scales of these shock compression processes. For that purpose, we observed the transformation mechanism of shock-compressed α-(Mg,Fe)₂SiO₄ into ringwoodite [ε-Mg₂SiO₄] in the solid state [3]. Ringwoodite is one of the most commonly-observed high-pressure polymorph mineral in the meteorites. We focused a high-power optical laser pulse into single crystal of α-Mg₂SiO₄ for inducing strong shock compression, where its transformation process was time-resolved by ultrafast diffractometry using SACLA x-ray free electron laser pulse of femtosecond time width. We found that a lattice-shear mechanism proceeded within several nanoseconds, which was much faster than any previous estimation of solid-state structure transformation mechanism of silicate minerals.

It was demonstrated that the mechanism proceeds even during short-lived shocks equivalent to those induced by impacts of small (sub-kilometer-scaled) parent asteroids. The mechanism mostly works during shock releases, such that the peak shock pressures deduced from the existence of shear-induced olivine polymorphs could be seriously underestimated. Since shorter-duration compression events occur more frequently in the asteroid impacts, the shorter events could have more frequently recorded the fast mechanism. When we carefully search for such impact records in more of meteorites and asteroids, we will be able to reconstruct the evolution history of early solar system through numerous muti-scale impact events.   

Peculiar melting of diamond on the Hugoniot

Laser shock experiments using the SG III proto-type facility were performed to study the melting of diamond over a range of shock pressure. We used the standard techniques for laser shock experiments of line-VISAR to measure shock velocity and SOP to observe the brightness. For a reference material quartz was set together with diamond sample to monitor the shock velocity and brightness during the decay shock. The Hugoniot of diamond has been determined through the impedance match method and the results were compared with those from the previous studies to be very similar. However, the temperature, calculated by the SOP data and the reflectivity from the VISAR measurement, is much lower than the previous one. The measured reflectivity appears above a shock velocity of ~20 km/s and changes with the optical variations of the shock front with increasing shock velocity. These changes of temperature and pressure on the Hugoniot are compared with available theoretical simulations. We discuss the diamond melting along the Hugonuot to be under equilibrium or not. Furthermore, the observed melting behaviors for shocked diamond provides new critical constrain on theoretical calculation and will affect prospect of understanding carbon in the interiors of large ice giants and design of first shock drive for ICF ignition process.