A compositional tipping point governing the mobilization and eruption style of rhyolitic magma
A compositional tipping point governing the mobilization and eruption style of rhyolitic magma
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DOI：
10.1038/nature24488
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发表时间：
2017-12-14
期刊：
影响因子：
64.8
通讯作者：
Dingwell, D. B.
中科院分区：
文献类型：
--
作者：
Di Genova, D.;Kolzenburg, S.;Dingwell, D. B.
地球上最粘稠的火山熔体和最大的爆炸性喷发（1）由钙碱性流纹岩（2，3）组成。这些火山爆发有可能影响全球气候（4）。喷发产物通常非常贫结晶且高度脱气，但岩浆大部分以结晶浆形式储存，其中含有少量含水量较高的间隙熔体（5）。目前还不清楚岩浆泥是如何被动员起来创造出大量可喷发的无晶体岩浆的。此外，流纹岩喷发（6-8）可以在喷发和爆发式喷发之间反复转换，这种转换很难归因于水含量或结晶度的流变效应（9，10）。在这里，我们测量的粘度的一系列熔体跨越黄石火山系统的组成范围，并发现，在一个狭窄的组成区，熔体粘度增加了两个数量级。这些粘度变化不是由当前的粘度模型（11，12）预测的，而是由熔体结构重组引起的，如通过拉曼光谱所证实的。我们确定了一个关键的成分转折点，独立记录在全球地球化学记录的流纹岩，流纹岩熔体的结晶或结晶，并明确区分从世界各地的爆发性存款的溢出。尽管自然喷发所固有的水含量和温度等其他参数可变，但熔体结构、粘度和喷发行为之间的这种相关性仍然存在。热力学建模演示了如何观察到的微妙的组成变化，导致流化或硬化的熔体可以诱导从熔体中的晶体生长或氧逸度的变化。然而，水和晶体含量的流变学效应不能单独解释成分和喷发风格之间的相关性。我们的结论是，钙碱性rhythmia的组成是决定性的，在确定地球上最大的火山系统的动员和喷发动力学，从而更好地了解如何熔体结构控制火山过程。
The most viscous volcanic melts and the largest explosive eruptions(1) on our planet consist of calcalkaline rhyolites(2,3). These eruptions have the potential to influence global climate(4). 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(5). It is unclear how magma mushes are mobilized to create large batches of eruptible crystal-free magma. Further, rhyolitic eruptions(6-8) 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(9,10). 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(11,12) 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.