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.

中文翻译：

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控制流纹质岩浆流动和喷发方式的组成临界点

我们星球上最粘稠的火山熔体和最大的爆发性喷发由钙碱性流纹岩组成。这些火山喷发有可能影响全球气候。喷发产物通常晶体含量极低且脱气率高，但岩浆大多以晶体糊状形式储存，其中含有少量含水量较高的间隙熔体。目前还不清楚岩浆糊是如何被动员起来产生大量可喷发的无晶体岩浆的。此外，流纹岩喷发可以在喷发和爆发式喷发之间反复转换，这种转变很难归因于水含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，发现在狭窄的成分区中，熔体粘度最多增加两个数量级。这些粘度变化不是由当前的粘度模型预测的，而是由熔体结构重组引起的，如拉曼光谱所证实的那样。我们确定了一个关键的成分临界点，在流纹岩的全球地球化学记录中独立记录，在该点流纹岩熔体流化或变硬，并清楚地将世界范围内的喷发性沉积物与爆炸性沉积物区分开来。尽管自然喷发中固有的水含量和其他参数（如温度）可变，但熔体结构、粘度和喷发行为之间的这种相关性仍然存在。热力学模型展示了观察到的导致熔体流化或硬化的细微成分变化是如何通过熔体的晶体生长或氧逸度的变化引起的。然而，仅水和晶体含量的流变效应并不能解释成分与喷发方式之间的相关性。我们得出结论，钙碱性流纹岩的组成在决定地球上最大火山系统的动员和喷发动力学方面具有决定性作用，从而更好地了解熔体结构如何控制火山过程。