In-situ X-ray diffraction of forsterite ramp-compressed to terapascal pressures at the National Ignition Facility: Initial results

The discovery of abundant large, rocky exoplanets known as super-Earths has generated considerable interest in the behavior of planetary materials up to terapascal pressures. One of the most important building blocks of rocky planets is forsterite (Mg2SiO4). At pressures between ~0.14 and 0.5 TPa, the assemblage MgO (periclase) plus post-perovskite (MgSiO3) is expected to be stable in an Mg2SiO4 composition according to theoretical calculations. Above 0.5 TPa, MgO and MgSiO3 are further predicted to recombine into a novel, highly coordinated Mg2SiO4 phase [1]. Using a high-powered laser with advanced pulse-shaping capability at the National Ignition Facility (Lawrence Livermore National Laboratory), we have shocklessly compressed synthetic forsterite to peak pressures between 300 and 1000 GPa over timescales of ~30 ns. The starting samples were low-porosity, fine-grained polycrystals produced by vacuum sintering techniques [2]. The compressed sample was probed at near the time of peak compression using in-situ X-ray diffraction. Laser interferometry was used to measure wave profiles that constrain the pressure history. Our preliminary results indicate that sample diffraction peaks could be observed over the whole pressure range, indicating that forsterite remains crystalline when ramp compressed to terapascal pressures on these timescales. This contrasts with laser-based shock compression experiments in which polycrystalline forsterite was observed to become amorphous above ~0.08 TPa [3]. Changes in the observed diffraction patterns as a function of pressure are suggestive of polymorphic phase transitions on the timescale of our experiments. Possible interpretations of the diffraction patterns will be discussed. [1] K. Umemoto, R. M. Wentzcovitch, P. B. Allen, Science, 311, 5763, (2006). [2] S. Koizumi et al., Physics and Chemistry of Minerals, 37, 505-518, (2010). [3] D. Kim et al., Journal of Geophysical Research, 126, 1, (2020).