Andesitic magma degassing investigated through H2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)
Andesitic magma degassing investigated through H2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)
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在蒙特塞拉特岛（小安的列斯群岛）苏弗里耶尔山火山，通过 H2O 蒸气-熔融分配卤素来研究安山岩浆脱气
DOI：
10.1016/j.epsl.2008.02.014
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发表时间：
2008
影响因子：
5.3
通讯作者：
A. Michel
中科院分区：
文献类型：
--
作者：
B. Villemant;J. Mouatt;A. Michel
Soufrière Hills火山(SHV)的岩浆脱气具有几乎永久的SO2通量和HCl产生速率的特点，这主要取决于穹顶的生长速率。通过对1995年至2006年在圆顶上以及火山碎屑流和火山喷发沉积中收集的碎屑进行结构、H2O和卤素分析，研究了脱气过程。在H2O脱气过程中，熔体中的氯、溴和碘强烈亏损，没有明显的氯-溴-碘分馏，而氟几乎不受影响。在SHV喷发的所有岩浆都遵循从岩浆室到浅层(∼1千米，P∼20兆帕)的多步骤脱气路径。然而，从这个深度来看，喷流和爆炸的路径是不同的；火山喷发是闭合系统脱气的结果，而热流穹顶的增长是闭合系统脱气的结果，直到很浅的深度(≤200m，P∼5-2 Mpa)，然后是开放系统脱气的结果。CSD用H2O溶解定律、理想气体定律和卤素在流纹岩熔体和H2O蒸气之间的分配系数(DV−li)来模拟。OSD的气体损失特性采用瑞利定律进行模拟。脱气诱导结晶是通过结晶率和脱气率的比率来引入的，范围为150-500。DV−lClfor OSD的范围在50-300之间，随着熔体氯含量的增加而增加。对于CSD，较低的有效DV−LCL(∼20)归因于动力学效应。与火山喷发相比，穹顶形成活动对大气化学的影响更大，因为OSD在提取卤素方面要有效得多。模型表明，HCl通量可以很好地反映穹顶形成的喷发速率。模型气体组成与实际气体组成的比较表明，在SHV气体羽流中有很高的HBr-BrO转化率(BrO/总BrO∼1/3)。在初始熔体、火山碎屑和高温气体中，氯、溴和碘的脱气行为暗示了类似的氯/溴(∼160)和溴/碘(∼90)。与其他岛弧(∼2 5 0~30 0)相比，SHV值较低的Cl/Br值可归因于浅层喷发前Br值的富集。自1995年以来，SHV的几乎永久的穹顶挤压可能对区域大气产生了重大影响，因为非常有效的喷射式脱气和气体羽流中卤素向活性物质的高转化率。
Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO2flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H2O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H2O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth (∼1 km, P∼20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤200 m, P∼5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H2O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H2O vapour (dv−li). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. dv−lClfor OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective dv−lCl(∼20) is attributed to kinetic effects. Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br∼1/3) in the SHV gas plume. The degassing behaviour of Cl, Br and I implies similar Cl/Br (∼160) and Br/I (∼90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs (∼250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.