Degassing of Laacher See phonolite – an experimental study

高粘性富sio2流纹岩岩浆的H2O脱气驱动火山爆发。粘性较低的声母岩岩浆也倾向于爆发，尽管流体动力学可能有很大不同。距今12900年，艾菲尔（Eifel）声母岩Laacher See岩浆库爆发，喷出约6.3 km3岩浆，火山灰流破坏了1000多km2的景观。艾菲尔火山的重新爆发可能会威胁到欧洲中心成千上万的人和宝贵的基础设施。因此，有必要对含水Laacher See phonolite的脱气进行实验研究。由囊泡形成的流体与含水熔体的相分离是驱动爆炸性火山喷发的基本机制。减压实验表明，在~ 100 MPa的过饱和压力下，流体囊泡的形成和生长引发含水硅酸盐熔体的相分离。水的过饱和导致流纹岩熔体的相分离，这是由于减压速率依赖于亚稳态的水团簇的均匀成核。另外，正如最近提出的富含钾和含金属的声质体维苏威火山熔体，过饱和可能会在不稳定状态下触发减压速率独立的独立分解。然而，最近的研究留下了一些重要的问题：(1)哪些参数导致不同熔体成分（流纹岩与维苏威phonolite）脱气行为的差异？(2)维苏威火山熔体的减压速率独立脱气行为是独一无二的，还是代表了含水音质熔体的一般相分离机制，其组成可能存在显著差异？(3)温度在含水熔体分解中起什么作用？除了有可能深入了解Laacher See火山系统的脱气机制外，对富钠和过铝Laacher See phonolite的脱气行为进行实验研究是回答这些基本问题的理想选择。实验数据结合天然浮石的囊泡结构对比，为Laacher See火山及其他富钠声母质火山系统的风险评估提供了重要的基础。

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.