Magma heating by decompression-driven crystallization beneath andesite volcanoes
Magma heating by decompression-driven crystallization beneath andesite volcanoes
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DOI：
10.1038/nature05100
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发表时间：
2006-09-07
期刊：
影响因子：
64.8
通讯作者：
Humphreys, Madeleine
中科院分区：
文献类型：
--
作者：
Blundy, Jon;Cashman, Kathy;Humphreys, Madeleine
火山爆发是由岩浆中富含H2O的蒸汽的出溶作用驱动的（1）。喷发动力学涉及蒸汽泡的成核和生长以及结晶之间的复杂相互作用，当岩浆在火山下方上升时，会产生岩浆物理性质的高度非线性变化（2）。这使得爆炸性火山活动难以建模，最终难以预测。一个关键的未知因素是通过次火山系统上升的岩浆的温度变化，因为它在途中失去气体并结晶（3）。岩浆的热力学模型，脱气，但不结晶，表明冷却和加热都是可能的（4）。迄今为止，由于难以跟踪地表以下几公里处移动岩浆的温度变化，还不可能对这些替代方案进行评估。在这里，我们扩展了最近关于斜长石晶体中捕获的玻璃熔体包裹体的工作（5），以开发一种跟踪两个活动安山岩火山下岩浆中压力-温度-结晶度路径的方法。我们使用溶解的H2O在熔融包裹体的压力，限制H2O的包裹体成为封闭的时候，不相容的微量元素浓度来计算相应的岩浆结晶度和斜长石熔体地质测温法来确定温度。这些数据与钛铁矿-磁铁矿地质测温法相关联，表明由于结晶潜热的释放，上升岩浆的温度增加了100摄氏度。这种加热可以解释安山质岩浆的几个常见的结构特征，否则可能被错误地归因于喷发前的岩浆混合。
Explosive volcanic eruptions are driven by exsolution of H2O-rich vapour from silicic magma(1). Eruption dynamics involve a complex interplay between nucleation and growth of vapour bubbles and crystallization, generating highly nonlinear variation in the physical properties of magma as it ascends beneath a volcano(2). This makes explosive volcanism difficult to model and, ultimately, to predict. A key unknown is the temperature variation in magma rising through the sub-volcanic system, as it loses gas and crystallizes en route(3). Thermodynamic modelling of magma that degasses, but does not crystallize, indicates that both cooling and heating are possible(4). Hitherto it has not been possible to evaluate such alternatives because of the difficulty of tracking temperature variations in moving magma several kilometres below the surface. Here we extend recent work on glassy melt inclusions trapped in plagioclase crystals(5) to develop a method for tracking pressure-temperature-crystallinity paths in magma beneath two active andesite volcanoes. We use dissolved H2O in melt inclusions to constrain the pressure of H2O at the time an inclusion became sealed, incompatible trace element concentrations to calculate the corresponding magma crystallinity and plagioclase-melt geothermometry to determine the temperature. These data are allied to ilmenite-magnetite geothermometry to show that the temperature of ascending magma increases by up to 100 degrees C, owing to the release of latent heat of crystallization. This heating can account for several common textural features of andesitic magmas, which might otherwise be erroneously attributed to pre-eruptive magma mixing.