Magma properties at deep Earth’s conditions from electronic structure of silica

doi: 10.7185/geochemlet.1902 SiO2 is the main component of silicate melts and thus controls their network structure and physical properties. The compressibility and viscosities of melts at depth are governed by their short range atomic and electronic structure. We measured the O K-edge and the Si L2,3-edge in silica up to 110 GPa using X-ray Raman scattering spectroscopy, and found a striking match to calculated spectra based on structures from molecular dynamic simulations. Between 20 and 27 GPa, [4]Si species are converted into a mixture of [5]Si and [6]Si species and between 60 and 70 GPa, [6]Si becomes dominant at the expense of [5]Si with no further increase up to at least 110 GPa. Coordination higher than 6 is only reached beyond 140 GPa, corroborating results from Brillouin scattering. Network modifying elements in silicate melts may shift this change in coordination to lower pressures and thus magmas could be denser than residual solids at the depth of the core-mantle boundary. Received 8 October 2018 | Accepted 21 December 2018 | Published 6 February 2019 1. Bayerisches Geoinstitut, University of Bayreuth, Bayreuth, D-95490, Germany 2. European Synchrotron Radiation Facility, 71 Avenue des Martyrs, Grenoble 38043, France 3. Fakultät Physik / DELTA, Technische Universität Dortmund, D-44221 Dortmund, Germany 4. Institut für Geowissenschaften, Universität Potsdam, Potsdam, Germany 5. Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada * Corresponding author (email: sylvain.petitgirard@erdw.ethz.ch) Letter The entrainment or settling of silicate melts in the deep Earth is related to their physical properties, such as density and viscosity. These properties are linked to the atomic structure that controls their ascent or settling in the deep mantle. SiO2 is the main component of silicate melts and is often used as a reference model to compare with the behaviour of other amorphous silicate compounds (Murakami and Bass, 2010) and melts at high pressure (Sanloup et al., 2013) because of its network forming nature in silicate magmas. Most of the data on SiO2 glass at high pressure have been obtained by X-ray diffraction (XRD) and show a change in the average coordination number (CN) from fourto six-fold starting at ~20 GPa and completing at pressures as low as 40 GPa (Benmore et al., 2010; Sato and Funamori, 2010). Between 40 and 130 GPa, it is unclear whether the CN increases above 6 (Prescher et al. 2017) or plateaus around 6 (Sato and Funamori, 2010) before increasing further above 140 GPa (Wu et al., 2012). Brillouin spectroscopy measurements also suggest an increase in velocities, possibly related to a CN higher than six-fold, at pressures above ~140 GPa for SiO2, 130 GPa for MgSiO3, and 110 GPa for Al-rich silicates (Murakami and Bass, 2010, 2011; Ohira et al., 2016). Many studies reported significant structural changes in silicate glasses at high pressure with potentially similar Si coordination changes in glasses and melts (Karki et al., 2007; Benmore et al., 2010; Sato and Funamori, 2010; Wu et al., 2012; Sanloup et al., 2013). Such changes seem also independent of the measurement time as illustrated by the first sharp diffraction peak position measured within hours (Sato and Funamori, 2010), minutes (Benmore et al., 2010; Prescher et al., 2017), or even a few seconds (Sanloup et al., 2013). Similarly, density measurements of SiO2 glasses are in very good agreement with each other, regardless of the starting glass material (Meade and Jeanloz, 1987; Petitgirard et al., 2017), and are also in agreement with the quenched liquid from molecular dynamic (MD) simulations (Wu et al., 2012). Still, the mechanisms associated with such high densification and compressibility (Petitgirard et al., 2017) and their link to local structural changes, remains debated (Sato and Funamori, 2010; Prescher et al., 2017) and requires more precise investigations on the changes in local structure with pressure. The discrepancy in CN evolution arises because of the use of non-elemental specific probes and due to the fact that XRD and Brillouin spectroscopy measurements only give access to the bulk structure and cannot probe the electrons directly involved in the chemical bonds that reflect structural