Degassing of Laacher See phonolite – an experimental study

高粘性富含二氧化硅的流纹质岩浆的H2O脱气作用推动了火山爆发。粘性较低的发泡石质岩浆也倾向于爆炸喷发，尽管流体动力学可能有很大不同。距今12900年前，埃菲尔火山的发音岩浆室喷发，喷出了约6.3km~3的岩浆，火山灰流破坏了超过1,000平方公里的地貌。埃菲尔火山爆发的重新启动可能会威胁到欧洲心脏地带的数千人和宝贵的基础设施。因此，有必要对含水Laacher SEE声石的脱气进行实验研究。流体与含水熔体之间的相分离是导致火山爆发的根本原因，减压实验表明，要使含水硅酸盐熔体通过形成和生长液泡而开始相分离，需要约100 Mpa的过饱和压力。流纹岩熔体中H2O的过饱和是通过亚稳态H2O团簇在减压速率下的均匀成核而导致相分离的。或者，正如最近对富钾和含铝声学维苏威熔体提出的那样，过饱和可能会在不稳定状态下触发与减压速率无关的调幅分解。然而，最近的研究留下了重要的问题：(1)哪些参数导致不同熔体成分(流纹岩和维苏威声石)的脱气行为不同？(2)维苏威熔体的脱气行为与减压速率无关是独特的，还是代表了成分可能显著不同的含水声质熔体的一般相分离机制？(3)温度在含水熔体的分解中起什么作用？除了有可能深入了解Laacher See火山系统的脱气机制外，对富钠过铝Laacher See声石的脱气行为的实验研究也是回答这些基本问题的理想选择。这些实验数据结合天然浮石的气泡结构对比，是拉契尔西火山和其他富钠语音橄榄火山系统风险评价的重要基础。

The H2O degassing of highly viscous SiO2-rich rhyolitic magma drives explosive volcanic eruptions. Less viscous phonolitic magma also tends to erupt explosively, although the fluid dynamics may be significantly different. The explosive eruption of the phonolitic Laacher See magma chamber 12900 years BP in the Eifel, which ejected ~ 6.3 km3 magma, devastated more than 1000 km2 of landscape by ash flows. A reactivation of the explosive Eifel volcanism could threaten thousands of people and valuable infrastructure in the heart of Europe. Therefore, it is necessary to experimentally investigate the degassing of hydrous Laacher See phonolite. The phase separation of a fluid from a hydrous melt by vesicle formation is a fundamental mechanism that drives explosive volcanic eruptions.Decompression experiments show that a supersaturation pressure of ~ 100 MPa is required to initiate phase separation of hydrous silicate melt by formation and growth of fluid vesicles. Supersaturation of H2O causes phase separation in rhyolitic melt by decompression rate dependent homogeneous nucleation of H2O clusters in the metastable state. Alternatively, as recently proposed for a potassium-rich and metaluminous phonolitic Vesuvius melt, supersaturation may trigger decompression rate independent spinodal decomposition in the unstable state. However, recent studies have left open important questions: (1) Which parameters cause the difference in degassing behavior of different melt compositions (rhyolite vs. Vesuvius phonolite)? (2) Is the decompression rate independent degassing behavior of the Vesuvius melt unique or does it represent a general phase separation mechanism of hydrous phonolitic melts that can differ significantly in their composition? (3) What role does temperature play in the decomposition of hydrous melts? Besides the possibility to gain insight into the degassing mechanisms of the Laacher See volcanic system, experimental investigation of the degassing behavior of the sodium-rich and peraluminous Laacher See phonolite is ideal to answer these fundamental questions. The experimental data in combination with the comparison of the vesicle textures of the natural pumice are important basics for the risk assessment of the Laacher See volcano and other sodium-rich phonolitic volcanic systems.