School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK.
Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Theresienstraße 41/III, 80333 München, Germany.
Nature. 2017 Dec 13;552(7684):235-238. doi: 10.1038/nature24488.
The most viscous volcanic melts and the largest explosive eruptions on our planet consist of calcalkaline rhyolites. These eruptions have the potential to influence global climate. The eruptive products are commonly very crystal-poor and highly degassed, yet the magma is mostly stored as crystal mushes containing small amounts of interstitial melt with elevated water content. It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions can switch repeatedly between effusive and explosive eruption styles and this transition is difficult to attribute to the rheological effects of water content or crystallinity. Here we measure the viscosity of a series of melts spanning the compositional range of the Yellowstone volcanic system and find that in a narrow compositional zone, melt viscosity increases by up to two orders of magnitude. These viscosity variations are not predicted by current viscosity models and result from melt structure reorganization, as confirmed by Raman spectroscopy. We identify a critical compositional tipping point, independently documented in the global geochemical record of rhyolites, at which rhyolitic melts fluidize or stiffen and that clearly separates effusive from explosive deposits worldwide. This correlation between melt structure, viscosity and eruptive behaviour holds despite the variable water content and other parameters, such as temperature, that are inherent in natural eruptions. Thermodynamic modelling demonstrates how the observed subtle compositional changes that result in fluidization or stiffening of the melt can be induced by crystal growth from the melt or variation in oxygen fugacity. However, the rheological effects of water and crystal content alone cannot explain the correlation between composition and eruptive style. We conclude that the composition of calcalkaline rhyolites is decisive in determining the mobilization and eruption dynamics of Earth's largest volcanic systems, resulting in a better understanding of how the melt structure controls volcanic processes.
地球上最粘稠的火山熔体和最大的爆发性喷发都由钙碱性流纹岩组成。这些喷发有可能影响全球气候。喷发产物通常晶体含量非常低，高度脱气，但岩浆主要以含有少量间质熔体且含水量较高的晶体糊的形式储存。目前尚不清楚岩浆糊是如何被移动以产生大量可喷发的无晶体岩浆的。此外，流纹质喷发可以在喷发和爆炸喷发之间反复切换，这种转变很难归因于含水量或结晶度的流变效应。在这里，我们测量了一系列跨越黄石火山系统成分范围的熔体的粘度，发现在一个狭窄的成分区域内，熔体粘度增加了两个数量级。这些粘度变化不能用当前的粘度模型来预测，而是由拉曼光谱证实的熔体结构重组引起的。我们确定了一个关键的成分转折点，该转折点在全球流纹岩的地球化学记录中有独立的记录，这使得流纹质熔体能够在全球范围内从喷发转变为爆发。尽管在自然喷发中存在变量水含量和其他参数（如温度），但这种熔体结构、粘度和喷发行为之间的相关性仍然存在。热力学模型表明，熔体结构的细微成分变化如何导致熔体的流化或变硬，这可以通过熔体中的晶体生长或氧逸度的变化来诱导。然而，水和晶体含量的流变效应本身并不能解释成分与喷发类型之间的相关性。我们得出结论，钙碱性流纹岩的成分对于决定地球最大火山系统的移动和喷发动力学至关重要，从而更好地理解熔体结构如何控制火山过程。
