Fig. 6
From: Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa
Inflection pressures in the P-V S profiles of the silicate glasses. SA1 (blue square, this study), SA2 (light blue diamond, this study), SiO2 glass (red circle, Murakami and Bass 2010), and MgSiO3 glass (green triangle, Murakami and Bass 2011) are shown in the left graph. This is regarded as the onset of the average Si-O coordination number increasing from 6 to 6+ (“6 + -fold (Si)” in this figure) in each glass (see Murakami and Bass 2010, 2011). The solid black line shows the possible transitional boundary at which the average Si-O coordination number increases from 6 to 6+ in SiO2-Al2O3 glasses, calculated using a linear leasts squares method based on the plots of SiO2 glass, SA1, and SA2. The pressure-intercept is fixed at 140 GPa, derived from the inflection pressure of SiO2 glass reported by Murakami and Bass (2010). Assuming an Al2O3 content of 13 mol% in an SiO2-Al2O3 glass, the average Si-O coordination number is expected to exceed 6 at 124 GPa (purple lines), which corresponds to a depth of 2690 km, much shallower than the CMB. The dashed gray line shows the boundary at which the average Si-O coordination number increases from just 6 to 6+ in MgSiO3 glass. The picture on the right is a schematic image of the accumulation of “dense” partial melts having structures governed by 6+-fold Si, generated from MORBs