Mobilisation of magmatic mushes: an experimental study with implications for large-scale volcanic eruptions

火山爆发是一个重大的社会和经济危害。岩浆的喷发是岩浆从地壳储层上升到地表的结果。喷发的岩浆的对应物是完全结晶的岩浆体（或“花岗岩体”），它们从未到达地表，而是完全在地壳中结晶。了解地壳岩浆体的命运很重要，因为地球上观测到的最大喷发（例如科罗拉多岛的鱼峡谷凝灰岩，体积>5000立方公里）是次火山岩浆体再活动的结果。几个巨大的火山，如黄石公园（美国）和乌图伦库（玻利维亚）已被确定为大规模喷发的潜在候选者。岩浆在地壳中停留期间冷却，结果结晶。晶体含量的增加已被证明会增加岩浆的粘度（物质在应力作用下流动的阻力）。相反，与在冷却储层底部注入热的、富含挥发性的岩浆有关的热量和/或挥发性物质（如H2O）的加入可导致晶体的再熔融、粘度的降低和深圳湾岩体的再活动。尽管这些过程在自然界中广泛发生，并且它们对于触发大爆发的重要性不言而喻，但由于缺乏专门针对这一问题的适当实验数据集，因此不可能对其进行量化。两个主要因素限制了我们预测地壳中岩浆体命运的能力：a）缺乏精确表征固体和液体相对比例随温度和含水量变化的岩石学实验，以及B）缺乏描述不同大小和形状晶体混合物粘度变化的适当流变（或“流动”）模型。本研究旨在使用高温高压设备重建岩浆的化学演化作为温度和含水量的函数，准确表征实验运行的产品，并使用模拟材料再现其特有的物理性质。测量粘度随晶体含量和形状的变化将把岩浆浆的化学演化与其流变学性质联系起来。岩石学和流变学实验相结合将使人们能够了解岩浆如何从深处上升到地表，并量化在岩浆泥重新活动之前需要供应的富含挥发物的热岩浆的数量。这将是极端的兴趣在火山像黄石和Uturuncu的规模和速度的隆起提供信息的数量和速度的岩浆供应的基础上的岩浆水库。将进行更多的实验，以确定地震波通过含有不同比例和形状的晶体的样品的传播速度。这些数据可用于反演从自然地震中获得的地震数据，以确定和描述活火山区下方的岩浆储层。将压缩波和剪切波传播结果与岩石学和流变学结果相结合，将有可能评估与活火山区下方部分熔融岩浆体存在相关的风险。

Volcanic eruptions constitute a major societal and economic hazard. Eruptions of silicic magmas are the result of magma ascent from crustal reservoirs to the surface. The counterparts of erupted silicic magmas are fully crystallized bodies of magma (or 'granitic plutons') that never reached the surface and crystallized totally in the crust. The knowledge of the destiny of crustal magma bodies is important because the largest eruptions observed on the Earth (e.g. Fish Canyon Tuff, Coloradao, volume>5000 km3) are the result of the re-mobilization of sub-volcanic magmatic bodies. Several giant volcanoes such as Yellowstone (USA) and Uturuncu (Bolivia) have been identified as potential candidates for large volume eruptions. During their residence in the crust magmas cool and, as a consequence, crystallize. The increase of crystals content has been demonstrated to increase magma viscosity (the resistance of a material to flow under the application of a stress). On the contrary, addition of heat and/or volatiles, such as H2O, related to injection of hot, volatile rich magma at the base of the cooling reservoir can lead to re-melting of the crystals, decrease of viscosity and re-mobilization of plutons. Despite the widespread occurrence of these processes in nature, and their undoubted importance for triggering major eruptions, no quantification of them is possible due to the lack of appropriate experimental datasets dedicated to this problem. Two main factors limit our ability to forecast the destiny of a magmatic body residing in the Earth crust: a) the lack of petrological experiments characterizing accurately the relative proportions of solid and liquid as function of temperature and water content and b) the lack of appropriate rheological (or 'flow') models describing the viscosity variations for mixtures of crystals with different sizes and shapes. This study aims to reconstruct the chemical evolution of magmas as functions of temperature and water content using high temperature and pressure apparatuses, characterize accurately the products of the experimental runs and reproduce their characteristic physical properties using analogue materials. The measurements of the variation of viscosity as function of crystal content and shape will link the chemical evolution of magmatic mushes with their rheological properties. The combination of petrological and rheological experiments will make it possible to understand how the rise of magmas from depth to the surface occurs and to quantify the amount of hot, volatile-rich magma that needs to be supplied before the remobilization of the magmatic mush occurs. This would be of extreme interest in volcanoes like Yellowstone and Uturuncu where the size and rate of the uplift give information on the amount and rate of magma supplied at the base of the magmatic reservoir. Additional experiments will be performed to relate the velocity of propagation of seismic waves through samples containing crystals in different proportions and shapes. These data can be used to invert the seismic data obtained from natural earthquakes to identify and characterize magmatic reservoirs present below active volcanic areas. The combination of the results obtained on propagation of compressional and shear waves with the petrological and rheological results will give the possibility to evaluate the risk associated with the presence of bodies of partially molten magma below active volcanic areas.