Special Collection: Glasses, Melts, and Fluids, as Tools for Understanding Volcanic Processes and Hazards. Experiments and models on H2O retrograde solubility in volcanic systems
Special Collection: Glasses, Melts, and Fluids, as Tools for Understanding Volcanic Processes and Hazards. Experiments and models on H2O retrograde solubility in volcanic systems
复制标题
特别收藏：玻璃、熔体和流体，作为了解火山过程和危害的工具。
DOI：
--
复制
发表时间：
2015
期刊：
影响因子：
--
通讯作者：
L. Porritt
中科院分区：
文献类型：
--
作者：
Amy G. Ryan;J. Russell;A. Nichols;K. Hess;L. Porritt
摘要本文介绍了在大气压下对来自冰岛克拉夫拉Hrafntinnuhrygur的流纹质黑曜石10 × 10 mm无夹套岩心进行的36个高温（900-1100 °C）实验。黑曜石是无气泡和无晶体的，H2O含量为0.11（4）wt%。将黑曜石芯加热到玻璃化转变温度（Tg）以上，保持0.25-24小时，然后淬火。在每个实验中，随着充满H2O蒸气的气泡成核和膨胀，样品的体积增加。独特的是，气泡没有在核心表面成核，也没有逃逸，在所有实验中保持质量。在每个等温实验套件内，芯的体积随着时间的推移而增加，直到它们达到最大值，之后继续加热不会引起体积的变化（通过He测比重法测量）。我们解释这些T-T条件代表熔体和出溶蒸汽之间的热化学平衡。这些实验是仿照恢复1大气压，温度依赖的流纹岩熔体中的水的溶解度。我们的结果定义了逆溶解度的大小（-7.1 × 10- 3wt%H2O/100 °C），并提供了H2O出溶反应的焓和熵的估计[ΔH° = 17.8 kJ/mol，ΔS° = 107 J/（K·mol）]。最后，我们通过建模逆行溶解度的玻璃化转变温度（Tg）的冷却火山系统在有关火山管道和地球表面的压力的影响。所有火山系统都冷却;逆溶解度的作用是使熔体在等压冷却时通过H2O溶解再水合，从而降低Tg并扩大熔体窗口。最终，熔体在较高的H2O含量和较低的温度下淬火，其中等压逆行溶解度曲线“捕获”演变的Tg。
Abstract We present a suite of 36 high-temperature (900-1100 °C) experiments performed on 10 × 10 mm unjacketed cores of rhyolitic obsidian from Hrafntinnuhryggur, Krafla, Iceland, under atmospheric pressure. The obsidian is bubble- and crystal-free with an H2O content of 0.11(4) wt%. The obsidian cores were heated above the glass transition temperature (Tg), held for 0.25-24 h, then quenched. During each experiment the volume of the samples increased as H2O vapor-filled bubbles nucleated and expanded. Uniquely, the bubbles did not nucleate on the surface of the core, nor escape, conserving mass during all experiments. Within each isothermal experimental suite, the cores increased in volume with time until they reached a maximum, after which continued heating caused no change in volume (measured by He-pycnometry). We interpret these T-t conditions as representing thermochemical equilibrium between the melt and exsolved vapor. These experiments are modeled to recover the 1-atm, temperature-dependent solubility of water in the rhyolite melt. Our results define the magnitude of retrograde solubility (-7.1 × 10-3 wt% H2O per 100 °C) and provide estimates of the enthalpy and entropy of the H2O exsolution reaction [ΔH° = 17.8 kJ/mol, ΔS° = 107 J/(K·mol)]. We conclude by modeling the implications of retrograde solubility for the glass transition temperatures (Tg) of cooling volcanic systems at pressures relevant to volcanic conduits and the Earth’s surface. All volcanic systems cool; the effects of retrograde solubility are to allow melts to rehydrate by H2O dissolution as they cool isobarically, thereby depressing Tg and expanding the melt window. Ultimately, the melt is quenched at higher H2O contents and lower temperatures where the isobaric retrograde solubility curve “catches” the evolving Tg.