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.