Elastic moduli of rock glasses under pressure to 8 kilobars and geophysical implications

Abstract

Shear and longitudinal velocities were measured by the ultrasonic phase comparison method as a function of pressure to 8 kbar on synthetic glasses of basalt, andesite, rhyolite, and quartz composition and on natural obsidian. Velocities of most of the glasses decrease anomalously with pressure, but increasingly more‐normal behavior occurs with decrease in SiO2 content. The pressure derivatives of rigidity and bulk modulus increase linearly, from −3.39 to −0.26 and from −5.91 to +2.09, respectively, with decrease in SiO2 content from 100 to 49%. The change from negative to positive in the pressure derivatives of both moduli and observed at Poisson's ratio of about 0.25 is consistent with the Smyth model for the anomalous elastic behavior of glass. If the temperature in the upper mantle is about 1500°C, tholeiitic basalt would be molten in accordance with the partial melt explanation for the low‐velocity zone; at 1300°C and below, basalt would be in the glassy state, especially if more felsic than tholeiite. At a temperature of 1370°C and at 30 kbar, reasonable values for the upper mantle at 100 km depth, the basalt glass of this study would have a viscosity of about 1013 P. On the basis of the theory of viscoelasticity the glass would support shear wave propagation at frequencies above 0.1 Hz. Under such conditions of PT, 10 to 30% basalt glass in a matrix either of eclogite or olivine would reduce the seismic velocities by 3 to 9% and could also account for the values observed in the low‐velocity zone.

High-pressing preparation of rock glasses and their elastic properties

“…The symbol of '' represents the data of artificial glasses described in the studies of Xu Jian and Manghnani (1992). The solid diamonds and triangles represent the results of analysis for artificial glasses in this work and Meister's (1980) study, respectively. The curves are fitting results for all data.…”

“…The contents of Fe 2 O 3 determined in the studies of Matsushima (1981), Meister (1980) and Xu Jian and Manghnani (1992) are equal to FeO data according to 4FeO+O 2 =2Fe 2 O 3 . The open triangles and squares represent the results of analysis for natural glasses in Meister's (1980) and Matsushima's (1981) studies, respectively. The symbol of '' represents the data of artificial glasses described in the studies of Xu Jian and Manghnani (1992).…”

“…Obsidian and basalt glasses are main glassy rocks in the nature, which were measured in previous studies (Matsushima, 1981(Matsushima, , 1989Meister et al, 1980;Pan Yucheng et al, 1998). Because there are some crystal minerals in the naturally-occurring glassy rocks, scientists prefer to study artificial glasses (Chou Shunruo et al, 1987);Matsushima, 1981;Meister et al, 1980;Xu Jian and Manghnani, 1992). But in previous studies, the glasses were synthesized at ambient pressure, which can not represent the synthetic products obtained under high temperature and high pressure conditions in the Earth's interior.…”

Effects of thermal quenching on mechanical properties of pyroclasts

“…5 shows K max as a function of H. Physically, H represents the volume to surface ratio. We assume a thermal conductivity of 1.6 W m − 1°C− 1 a value appropriate for rhyolite glass (Bagdassarov and Dingwell, 1994), initial temperature difference of 500°C, v = 0.25 (Meister et al, 1980), E = 70 GPa (Meister et al, 1980), α =5 × 10 − 5°C− 1 (Spera, 2000). Heat transfer coefficients of 2.5 × 10 3 and 1 × 10 5 W m − 2°C− 1 bound the yellow region, representing convection with phase change of boiling (Incropera and DeWitt, 2002).…”

“…For typical values of E = 70 GPa (Meister et al, 1980), v = 0.25 (Meister et al, 1980), ΔT = 500°C, and α = 5 × 10 −5 /°C (Spera, 2000), these stresses will be 2.3 GPa. This is greater than the tensile strength of volcanic glasses, which is about 0.1 GPa (Webb and Dingwell, 1990).…”