The influence of H2O and CO2 on the glass transition temperature: insights into the effects of volatiles on magma viscosity
The influence of H2O and CO2 on the glass transition temperature: insights into the effects of volatiles on magma viscosity
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DOI：
10.1127/0935-1221/2007/0019-1751
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发表时间：
2007-10
影响因子：
2.1
通讯作者：
Y. Morizet;A. Nichols;S. Kohn;R. Brooker;Donald Bruce Dingwell
中科院分区：
文献类型：
--
作者：
Y. Morizet;A. Nichols;S. Kohn;R. Brooker;Donald Bruce Dingwell
CO2在喷发过程中起着重要的作用;特别是，它有可能在比H2O低的浓度下达到饱和并启动脱气。这种CO2损失对岩浆粘度的影响没有得到很好的限制，特别是与H2O损失的既定影响相比。在理解CO2溶解机制方面，最近的光谱研究表明，CO2的形态是强烈的温度依赖性，并且在低于Tg的淬火玻璃中保存的CO2形态不同于熔体中观察到的真实CO2形态。然而，CO2对玻璃化转变温度的影响，并通过推断粘度，以前没有建立。本文通过对含H2O和CO2的响岩和硬玉玻璃进行量热测量，定义了单一的玻璃化转变温度（Tg起始），研究了挥发分对玻璃化转变区间的影响。样品在1300至1550 °C之间、在1.0至2.5GPa下在活塞-气缸装置中合成，并且含有至多2.29重量%的二氧化碳和高达5.49重量% H 2 O.对于这两种组合物，H2 O有很大的影响，在降低Tg的开始，但CO2似乎有很少或没有影响。在整个H2O含量范围内，响岩和硬玉的Tg起始温度分别在870 ~ 523 K和1036 ~ 636 K随H2O含量呈指数下降，与CO2含量无关。没有可测量的影响，CO2对Tg的开始观察。这些结果表明，与H 2 O相比，CO 2对熔体物理性质的变化贡献很小。它们也提供了强有力的证据，脱钩的CO2形态从散装硅酸盐熔体结构弛豫过程中的Tg。
CO 2 can play an important role in eruptive processes; in particular, it has the potential to reach saturation at lower concentrations than H 2 O and initiate degassing. The effect of such CO 2 loss on magma viscosity is not well constrained, especially compared to the established effects of H 2 O loss. In terms of understanding the CO 2 solubility mechanism, recent spectroscopic studies have shown that CO 2 speciation is strongly temperature dependent and that CO 2 speciation preserved in quenched glasses below T g is different from the true CO 2 speciation observed in the melts. However, the effect of CO 2 on the glass transition temperature, and by inference the viscosity, has not been previously established. In this study, calorimetric measurements were conducted on synthetic H 2 O- and CO 2 -bearing phonolite and jadeite glasses in order to investigate the volatile’s effect on the glass transition interval, by defining a single glass transition temperature ( T g onset ). The samples were synthesised in a piston-cylinder apparatus between 1300 and 1550 °C, at 1.0 to 2.5 GPa, and contained up to 2.29 wt.% CO 2 and up to 5.49 wt.% H 2 O. For both compositions, H 2 O has a large effect in reducing T g onset , but CO 2 appears to have little or no effect. For the entire range of H 2 O contents, T g onset decreases exponentially with H 2 O content from 870 to 523 K and 1036 to 636 K for phonolite and jadeite, respectively, regardless of the CO 2 content. No measurable effect of CO 2 on T g onset was observed. These results suggest that compared to H 2 O, CO 2 contributes little to changes in the physical properties of the melt. They also provide strong evidence for the decoupling of CO 2 speciation from the bulk silicate melt structural relaxation process at T g .