Modelling the dynamics and thermodynamics of volcanic degassing
Modelling the dynamics and thermodynamics of volcanic degassing
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DOI：
10.1007/s004450050234
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发表时间：
1998-12-01
影响因子：
3.5
通讯作者：
Blake, S
中科院分区：
文献类型：
--
作者：
Stevenson, DS;Blake, S
从火山被动脱气率进行了研究，通过模拟对流翻转的密集脱气和不太密集的富含气体的岩浆在一个垂直管道连接浅脱气区与深岩浆房。实验室实验被用来限制我们的理论模型的翻转率，并阐述了Kazahaya等人（1994）提出的模型。我们还介绍了二氧化碳饱和的深室和绝热冷却上升岩浆的影响。我们发现，翻转发生在岩浆沿着管道的同心流动，虽然流动的细节取决于岩浆的粘度比。在对流翻转限制富气岩浆供应的情况下，气体排放命运与翻转岩浆的流速成比例（与驱动对流的密度差成比例，管道半径的四次方。并且与脱气的MAGMA粘度成反比）和脱气的水的质量分数。有效的脱气作用增强了密度差，但也增加了岩浆的粘性，从而抑制了对流。模拟了两个脱气火山。在斯特龙博利，假设一个2公里深，30%的结晶玄武岩室，含有0.5重量%溶解水，类似于700 kg s（-1）的岩浆水通量可以用4-10 m半径的导管模拟，脱气20-100%的可用水和所有1 - 4 vol. %二氧化碳室气体。1980年6月在圣海伦斯山，假设有一个7公里深、含39%晶体的英安岩室，溶解水，类似于500 kg s（-1）的岩浆水通量可以用半径为22-60 m的导管模拟，脱气类似于2-90%的可用水和0.1 - 3体积%的所有可用水。二氧化碳毒气室。这些结果的范围与以前的模型和观测结果一致。由脱气驱动的对流提供了一种将挥发物从深层岩浆房转移到大气中的合理机制，它可以解释在许多持续活跃的火山中测量到的气体通量。
The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO2-saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission fate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power. and inversely proportional to the degassed mag ma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this damp ens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the similar to 700kg s(-1) magmatic water flux can be modelled with a 4-10 m radius conduit, degassing 20-100% of the available water and all of the 1 to 4 vol.% CO2 chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the similar to 500 kg s(-1) magmatic water flux can be modelled with a 22-60 m radius conduit, degassing similar to 2-90% of the available water and all of the 0.1 to 3 vol.% CO2 chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes.