Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes
Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes
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硅酸盐熔体的结构弛豫和地质过程中的非牛顿熔体流变学
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发表时间：
1989
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影响因子：
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通讯作者：
S. Webb
中科院分区：
文献类型：
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作者：
Donald Bruce Dingwell;S. Webb
硅酸盐熔体中结构弛豫的时间尺度定义了从液体（弛豫）到玻璃态（非弛豫）行为的转变。硅酸盐熔体中的结构弛豫可以用弛豫时间τ来描述，这与体积弛豫和剪切弛豫的时间尺度具有相同的数量级的观察结果一致。显著不放松行为的开始发生在τ以上2 log 10单位的时间。在剪切松弛的情况下，松弛时间可以使用粘弹性材料的麦克斯韦关系式来量化; τS = ηS/G∞（其中τS是剪切松弛时间，G∞是无限频率下的剪切模量，ηS是零频率剪切粘度）。已知SiO2和其他几种硅酸盐玻璃的G∞值。每种玻璃的剪切模量G∞和体积模量K∞的大小相似，两种模量对温度和组成的变化相对不敏感。相比之下，硅酸盐熔体的剪切粘度在至少十个数量级的范围内，其中组成在固定温度下，并且温度在固定组成下。因此，相对于ηS，G∞可以被认为是常数。对于已知剪切粘度的大量硅酸盐熔体，由麦克斯韦关系式计算的弛豫时间与已有的频率依赖性的起始数据吻合得很好声速（色散），非牛顿粘度的开始，量热玻璃化转变的扫描速率依赖性，利用氧扩散跳跃的时间尺度和从29 Si NMR研究中获得的Si-O键交换频率。使用在一定频率和应变范围内获得的数据-速率，我们说明了在硅酸盐熔体的实验室实验中的松弛与非松弛行为的意义。同样，使用应变率估计岩浆过程中，我们评估的意义的液体玻璃化转变的火成岩成因。
The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, τ, consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log10 units of time above τ. In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; τS = ηS/G∞ (where τS is the shear relaxation time, G∞ is the shear modulus at infinite frequency and ηS is the zero frequency shear viscosity). The value of G∞ known for SiO2 and several other silicate glasses. The shear modulus, G∞, and the bulk modulus, K∞, are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to ηS, G∞ may be considered a constant (independent of composition and temperature) and the value of ηS, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known.For silicate melts, the relaxation times calculated from the Maxwell relationship agree well with available data for the onset of the frequency-dependence (dispersion) of acoustic velocities, the onset of non-Newtonian viscosities, the scan-rate dependence of the calorimetric glass transition, with the timescale of an oxygen diffusive jump and with the Si-O bond exchange frequency obtained from 29Si NMR studies.Using data obtained over a range of frequencies and strain-rates we illustrate the significance of relaxed versus unrelaxed behavior in laboratory experiments on silicate melts. Similarly, using strain-rate estimates for magmatic processes we evaluate the significance of the liquid-glass transition in igneous petrogenesis.