13:45 〜 15:15
[SMP27-P07] Pressure and composition dependence of elastic wave velocity of Al2O3–SiO2 glasses
キーワード：アルミノケイ酸塩ガラス、高圧、弾性波速度、TEM
Aluminum ion (Al3+) is considered to play an important role in densification of aluminosilicate glasses, structural and compositional analogs of magmas, at high-pressure conditions. For example, it has been reported that Al–O coordination number (CN) starts to increase at lower pressure condition than Si–O CN (e.g., [1]), implying that Al-rich glasses may undergoes densification at lower pressures than Al-poor glasses. However, most of the aluminosilicate glasses measured by high-pressure experiments have Si-rich compositions simulating natural rocks or minerals, and therefore the discussion about the role of Al in pressure-induced structural and elastic changes has been subject to the major network-former cation Si and other network-modifier cations. To address the detail of the role of Al in structural and elastic changes under pressure, we conducted the combined in situ X-ray and ultrasonic measurements on xAl2O3–(100−x)SiO2 glasses in a wide range of Al2O3 content (x = 0, 28, 36, 44, 50, and 60, mol%) synthesized by container-less heating method. After the ultrasonic measurements, we also performed TEM analyses on the recovered glass samples.
The ultrasonic measurements were carried out at the beamline BL04B1, SPring-8, and we successfully determined the VP and VS of the six Al2O3–SiO2 glass samples up to 24 GPa. At a given pressure point, the VP and VS of the Al2O3–SiO2 glasses increased with increasing Al2O3 content because in this binary system the increase of Al2O3 content strengthens bulk moduli of glass [2]. All the sample glasses showed velocity reduction corresponding to shrinkage of intermediate-range ordered structure (i.e., the reduction of interstitial voids space) under compression, but the pressure range and degree of the velocity reduction were significantly changed by Al2O3 content. The degree of velocity reduction became smaller with increasing Al2O3, and the velocity reductions of SiO2 (x = 0), 28Al2O3–72SiO2, and 36Al2O3–64SiO2 glasses ended at ~3.5 GPa, 5.0 GPa, and 6.5 GPa, respectively. The other Al-rich glasses have the velocity minima at ~6–7 GPa, almost same as 36Al2O3–64SiO2 glass. After velocity reduction was ended, the SiO2 glass showed linear increases in VP and VS up to 21 GPa, while the Al-bearing glasses showed rapid increase in velocities at ~7–11 GPa, and then shifted to gradual increase. The rapid increase of VP and VS between ~7–11 GPa became more distinct with increasing Al2O3 content.
Previous studies reported that the Al–O CN of 60Al2O3–40SiO2 glasses likely reached 6 at ~20 GPa [3], while the Si–O CN of SiO2 glass remained in 4 up to ~10 GPa and then increased to 6 at ~10–40 GPa [4, 5]. In addition, previous TEM analyses demonstrated the nanoscale immiscibility composed by Si-rich and Al-rich domains in the ~10 < x <~60 bulk-composition (e.g., [6]). Our TEM analyses showed that the immiscible textures were still preserved in the recovered samples, except for the two homogeneous glasses (x = 0, 60). High-coordinated AlOx units (AlO5 and AlO6) are likely dominant in Al-rich domains, and therefore the rapid velocity increase in the Al-rich glasses observed at ~7–11 GPa may be mainly originated by the rapid densification of Al-rich domains. This result may suggest that if a deep magma includes Al-rich domains in its structure it undergoes densification just above 410 km discontinuity.
In this presentation, we will also discuss the correlation between the size of Si-rich and Al-rich domains and the pressure point of velocity minima.
References
[1] B. Mysen & P. Richet. Silicate glasses and melts: second edition. Elsevier (2019).
[2] G. A. Rosales-Sosa et al. Sci. Rep. 6, 23620 (2016).
[3] I. Ohira et al. Geochem. Persp. Let. 10, 41–45 (2019).
[4] C. J. Benmore et al. Phys. Rev. B 81, 054105 (2010).
[5] T. Sato & N. Funamori. Phys. Rev. B 82, 184102 (2010).
[6] S. H. Risbud. J. Non-Cryst. Solids 49, 241–251 (1982).
The ultrasonic measurements were carried out at the beamline BL04B1, SPring-8, and we successfully determined the VP and VS of the six Al2O3–SiO2 glass samples up to 24 GPa. At a given pressure point, the VP and VS of the Al2O3–SiO2 glasses increased with increasing Al2O3 content because in this binary system the increase of Al2O3 content strengthens bulk moduli of glass [2]. All the sample glasses showed velocity reduction corresponding to shrinkage of intermediate-range ordered structure (i.e., the reduction of interstitial voids space) under compression, but the pressure range and degree of the velocity reduction were significantly changed by Al2O3 content. The degree of velocity reduction became smaller with increasing Al2O3, and the velocity reductions of SiO2 (x = 0), 28Al2O3–72SiO2, and 36Al2O3–64SiO2 glasses ended at ~3.5 GPa, 5.0 GPa, and 6.5 GPa, respectively. The other Al-rich glasses have the velocity minima at ~6–7 GPa, almost same as 36Al2O3–64SiO2 glass. After velocity reduction was ended, the SiO2 glass showed linear increases in VP and VS up to 21 GPa, while the Al-bearing glasses showed rapid increase in velocities at ~7–11 GPa, and then shifted to gradual increase. The rapid increase of VP and VS between ~7–11 GPa became more distinct with increasing Al2O3 content.
Previous studies reported that the Al–O CN of 60Al2O3–40SiO2 glasses likely reached 6 at ~20 GPa [3], while the Si–O CN of SiO2 glass remained in 4 up to ~10 GPa and then increased to 6 at ~10–40 GPa [4, 5]. In addition, previous TEM analyses demonstrated the nanoscale immiscibility composed by Si-rich and Al-rich domains in the ~10 < x <~60 bulk-composition (e.g., [6]). Our TEM analyses showed that the immiscible textures were still preserved in the recovered samples, except for the two homogeneous glasses (x = 0, 60). High-coordinated AlOx units (AlO5 and AlO6) are likely dominant in Al-rich domains, and therefore the rapid velocity increase in the Al-rich glasses observed at ~7–11 GPa may be mainly originated by the rapid densification of Al-rich domains. This result may suggest that if a deep magma includes Al-rich domains in its structure it undergoes densification just above 410 km discontinuity.
In this presentation, we will also discuss the correlation between the size of Si-rich and Al-rich domains and the pressure point of velocity minima.
References
[1] B. Mysen & P. Richet. Silicate glasses and melts: second edition. Elsevier (2019).
[2] G. A. Rosales-Sosa et al. Sci. Rep. 6, 23620 (2016).
[3] I. Ohira et al. Geochem. Persp. Let. 10, 41–45 (2019).
[4] C. J. Benmore et al. Phys. Rev. B 81, 054105 (2010).
[5] T. Sato & N. Funamori. Phys. Rev. B 82, 184102 (2010).
[6] S. H. Risbud. J. Non-Cryst. Solids 49, 241–251 (1982).