[S-IT17] Property and role of liquids inside terrestrial planets

[SIT17-01] Pressure-induced structural change of basaltic glass up to 18 GPa

*Tomonori Ohashi¹, Tatsuya Sakamaki¹, Ken-ichi Funakoshi², Takanori Hattori³, Naoki Hisano¹, Jun Abe², Akio Suzuki¹ (1.Department of Earth Science, Tohoku University, 2.Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society (CROSS), 3.J-PARC Center, Japan Atomic Energy Agency)

Keywords:silicate glass, oxygen packing, X-ray diffraction, neutron diffraction

The structures of cold-compressed basaltic glass were investigated at pressures of up to 18 GPa using in situ X-ray and neutron diffraction to acquire insights into the physicochemical properties of deep magmas. On compression, basaltic glass changes its compression behavior: the mean O-O coordination number (CN_OO) starts to rise with maintaining the mean O-O distance (r_OO) above about 2–4 GPa, and then CN_OO stops increasing and r_OO begins to shrink along with the increase in the mean coordination number of Al (CN_AlO) above ~9 GPa. The change around 9 GPa is interpreted by the change in the contraction mechanism from bending tetrahedral networks of glass to increasing oxygen packing ratio via the increase in CN_AlO. The analysis of the oxygen packing fraction (η_O) under high pressure revealed that η_O exceeds the value for dense random packing, suggesting that the oxygen-packing hypothesis recently proposed cannot account for the pressure-induced structural transformations of silica and silicate glasses. The rise of the CN_OO at 2–4 GPa reflects the elastic softening of fourfold-coordinated silicate glass, which may be the origin of anomalies of elastic moduli in basaltic glass at ~2 GPa previously reported by Liu and Lin (2014).
The widths of both the first sharp diffraction peak and the principal peak show contrastive compression behaviors between modified silicate and silica glasses. This result suggests that modified silicate glasses represent different pressure evolutions in the ranges of the intermediate- and the extended-range order structures from those of silica glass, likely due to the presence of modifier cations and the resultant formations of smaller rings and cavity volume.

Reference
Liu, J., and Lin, J.-F. (2014) Abnormal acoustic wave velocities in basaltic and (Fe,Al)-bearing silicate glasses at high pressures. Geophysical Research Letters, 41, 8832–8839.