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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Magma degassing and crystallization processes during eruptions of high-risk Neapolitan-volcanoes: Evidence of common equilibrium rising processes in alkaline magmas
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Magma degassing and crystallization processes during eruptions of high-risk Neapolitan-volcanoes: Evidence of common equilibrium rising processes in alkaline magmas

机译：高危那不勒斯火山喷发期间的岩浆脱气和结晶过程：碱性岩浆中常见平衡上升过程的证据

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Compositional, textural and experimental data on products from explosive and effusive eruptions of Neapolitan volcanoes (Campi Flegrei and Somma-Vesuvio) allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas. Significant differences in compositional and textural features have been recognized between lavas, scoria and pumice resulting respectively from effusive, moderately and extremely explosive eruptions. Pumice samples have highly-vesicular glassy matrix, low microlite number density and moderate to high water content. Crystal Size Distributions (CSD) are steep with high intercept values; the narrow microlite size range indicates single nucleation event. Scotia products are characterized by moderate vesicularity and water content. They have high number density of microlites which are bimodal in size. CSD show distinct inflections that are explained as two crystal populations growing in distinct time. Lava samples generally have low vesicularities, moderate to high microcrystalline groundmass and low glass water content. The comparison between textural and compositional features of natural rocks with samples obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near-equilibrium conditions even at high decompression rate. Moderate to long magma rise times, calculated in the order of a few days, produce open-degassing responsible for moderately explosive to effusive activity. Short magma rise times, calculated in the order of a few hours, result in closed-system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold. Vesicularity and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10-30 MPa. In these conditions, degassing, fragmentation and in turn the eruptive style is strongly influenced by initial conditions in the magma chamber (volatile content, temperature, pressure) instead of decompression rate, in contrast with that observed for rhyolitic melts. These differences have important consequences in terms of volcanic hazards and risk. The low-viscosity alkaline magma is able to maintain efficient degassing even during the final stage of magma ascent, favoring, in the case of closed-system, fragmentation and explosive activity. (c) 2006 Elsevier B.V. All rights reserved.
机译：那不勒斯火山（Campi Flegrei和Somma-Vesuvio）的爆发性喷发性喷发产物的成分，结构和实验数据使我们能够限制碱性岩浆喷发期间的脱气和破碎条件。熔岩，火山灰和浮石之间的成分和质地特征存在显着差异，分别是喷发性，中度和极度爆炸性喷发造成的。浮石样品具有高泡状的玻璃状基质，低的微晶石数密度和中等至高的水含量。晶体尺寸分布（CSD）陡峭，截距值高;窄的微晶尺寸范围表示单晶核化事件。斯科舍省产品的特征是适度的水泡性和水含量。它们具有双峰大小的高密度微晶。 CSD显示出不同的拐点，这可以解释为两个晶体在不同的时间增长。熔岩样品通常具有低水泡度，中等至高的微晶底质和低玻璃水含量。将天然岩石的结构特征和组成特征与通过减压实验获得的样品进行比较，可以得出结论：即使在高减压速率下，岩浆上升期间的脱气过程也会在接近平衡的条件下发生。按几天计算，中等至长的岩浆上升时间会产生疏松的脱气作用，导致中等程度的爆炸性至喷发性活动。较短的岩浆上升时间（按几个小时计算）会导致密闭系统脱气，当气泡增长量达到固定阈值时，爆炸性碎片便会发生。在浮石的基质玻璃上测量的泡状度和水含量表明该过程在10-30MPa的压力下发生。在这种情况下，与流纹熔体观察到的相反，脱气，破碎和喷发方式受岩浆室内初始条件（挥发物含量，温度，压力）的影响很大，而不是受减压速率的影响。这些差异对火山危害和风险具有重要的影响。低粘度碱性岩浆即使在岩浆上升的最后阶段也能保持有效的脱气，在封闭系统中有利于破碎和爆炸活动。 （c）2006 Elsevier B.V.保留所有权利。

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