In situ observations of phase changes in shock compressed forsterite

Creators: Newman, Matthew G.; Kraus, Richard; Akin, Minta C.; Bernier, Joel; Dillman, Amanda; Homel, Michael; Lee, Sally; Lind, Jonathan; Mosenfelder, Jed L.; Pagan, Darren; Sinclair, Nicholas; Asimow, Paul D.

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.

Additional Information

Plain Language Summary: Modeling the interior structure of rocky planets requires us to understand the properties of planetary materials at the pressures and temperatures that are relevant to their interiors and formation processes. To measure the properties of the Earth's mantle at these conditions, researchers have traditionally performed hypervelocity impact experiments, launching a bullet at 10,000 mph into the sample of interest, which creates a shock wave that both heats and compresses the sample. Scientists then measure the temperature, pressure, and density of the samples using advanced diagnostics, from which they can learn how these materials would respond within the Earth or other large rocky planets. However, these impact experiments last one millionth of a second and we must ask ourselves, does an experiment that lasts one millionth of a second represent how a material responds on the tens of thousands of years timescale during mantle convection? Here we utilize a new diagnostic to measure the crystal structure of shock compressed forsterite at conditions similar to those deep within Earth's mantle. We find that forsterite does not reach the same state in a millionth of a second as it would in ten thousand years, instead it reaches a non‐equilibrium, or metastable structure.

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