Degassing of Laacher See phonolite – an experimental study

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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.