MATTER AT EXTREMES

24 ESRF

In situ X-ray diffraction of silicate liquids and glasses under dynamic and static compression to megabar pressures, G. Morard (a,b), J.-A. Hernandez (c,d), M. Guarguaglini (c), R. Bolis (c), A. Benuzzi-Mounaix (c), T. Vinci (c), G. Fiquet (a), M.A. Baron (a), S.-H. Shim (e), B. Ko (e), A.E. Gleason (f,g), W.L. Mao (f), R. Alonso-Mori (g), H.J. Lee (g), B. Nagler (g), E. Galtier (g), D. Sokaras (g), S.H. Glenzer (g), D. Andrault (h), G. Garbarino (i), M. Mezouar (i), A.K. Schuster (j) and A. Ravasio (c), PNAS

117(22) 11981-11986 (2020); https://doi. org/10.1073/pnas.1920470117. (a) Sorbonne Université, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Museum National d Histoire Naturelle, CNRS, Paris (France) (b) Université Grenoble Alpes, CNRS, ISTerre, Grenoble (France) (c) Laboratoire pour l Utilisation des Lasers Intenses (LULI), Ecole Polytechnique, Palaiseau (France) (d) Centre for Earth Evolution and Dynamics, University of Oslo (Norway)

(e) School of Earth and Space Exploration, Arizona State University (USA) (f) Geological Sciences, Stanford University (USA) (g) SLAC National Accelerator Laboratory, Menlo Park (USA) (h) Université Clermont Auvergne, CNRS, LMV, Clermont-Ferrand (France) (i) ESRF (j) Helmholtz-Zentrum Dresden Rossendorf, Dresden (Germany)

PRINCIPAL PUBLICATION AND AUTHORS

performed to provide nanosecond resolution on silicate structural transformations. These experiments, performed at SLAC National Acceleratory Laboratory, consisted of using X-ray diffraction to probe the structure of dynamically compressed amorphous silicates up to the liquid state at 130 GPa and 6000 K.

This experimental dataset (Figure 11) was then compared with amorphous silicates compressed statically in diamond anvil cells (up to 157 GPa at room temperature) at high-pressure beamline ID27, using a multichannel collimator slit system to reduce the inelastic X-ray contribution from the diamond anvils. Finally, a series of theoretical calculations was performed to complement these data (Figure 11). Diffraction spectra of amorphous silicates, SiO2 or MgSiO3 for example, evolve drastically with increasing pressure, related to the change in coordination of oxygen atoms around silicon atoms, from four at ambient pressure to six above 20 GPa (Figure 11). Beyond this increase in Si coordination observed at 20 GPa, the results find no evidence for major structural changes occurring in the silicate melts studied up to pressures and temperatures exceeding Earth s core mantle boundary conditions, supported by molecular dynamics calculations.

In a more general frame, comparing the structure of the disordered silicates at similar densities, a common high-pressure structural evolution of glasses and liquid silicates was revealed (Figure 12). These findings reinforce the widely held assumption that the silicate glasses studies are appropriate structural analogues for understanding the atomic arrangement of silicate liquids at these high pressures.

Fig. 12: Peaks position of the main diffuse contribution of the X-ray diffraction patterns (illustrated in Figure 11) as a function of the density of the disordered silicate (MgSiO3 and SiO2).

Fig. 11: Diffraction patterns of compressed amorphous MgSiO3 obtained under (a) static and (b) dynamic compression, and (c) from molecular dynamics

simulations. Comparison for spectra at similar density (d) shows an excellent agreement for the position of the two main diffuse contributions.