In situ observations of phase changes in shock compressed forsterite

Abstract

The high-pressure response of Mg2SiO4 forsterite is important for modeling chemical stratification in the mantle. Previous shock recovery experiments on forsterite show discrepant results as to whether forsterite undergoes segregation into its equilibrium phase assemblage of compositionally distinct structures upon shock compression. Here, we present the results of plate impact experiments on polycrystalline forsterite conducted at the Dynamic Compression Sector of the Advanced Photon Source. In situ x-ray diffraction measurements were used to probe the crystal structure(s) in the shock state and to investigate potential decomposition into periclase and bridgmanite. In contrast to previous interpretations of the forsterite shock Hugoniot, we find that forsterite does not decompose, but instead reaches the forsterite III structure, which is a metastable structure of Mg2SiO4. This work has important implications for the phase(s) that are present behind the shock front (kinetic versus equilibrium) for material systems that may phase segregate.