Experimental constraints on permeable gas transport in crystalline silicic magmas
Experimental constraints on permeable gas transport in crystalline silicic magmas
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结晶硅岩浆中可渗透气体输运的实验约束
DOI：
10.1007/s00410-012-0750-8
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发表时间：
2012
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影响因子：
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et al
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作者：
Okumura S.;Nakamura M.;et al
岩浆中的气体和流体通过可渗透流通过互连的气泡网络进行传输，控制着火山管道中上升的岩浆的脱气速率以及岩浆储层的糊状边界层中的流体传输。因此，阐明其机制和速率对于理解火山喷发的爆炸性以及岩浆库的演化和动力学至关重要。最近的实验研究确定了无晶体流纹岩和玄武岩的气体渗透率。然而，还没有实验研究调查晶体含量对岩浆中可渗透气体传输的影响。在本研究中，我们对结晶度为 30 和 50 vol% 的含水流纹岩熔体进行了减压实验，以研究晶体对岩浆泡化过程中气泡微观结构和气体渗透率的影响。尺寸控制（100 目）刚玉晶体被用作硅质岩浆中斑晶的类似物。使用 X 射线 CT 的微观结构分析表明，随着减压后最终压力的降低，气泡聚结且其连通性增加，即气泡度的增加。只要无晶体基础中熔体部分的孔隙度（熔体孔隙度）相似，在熔体孔隙度<68 vol%时，就没有观察到结晶度对气泡聚结和连通程度的明显影响。刚玉与水性流体以及斜长石和碱性长石表现出大的接触角；这未能诱导气泡在其表面上有效的异质成核和聚结。所有运行产品的气体渗透率均低于当前分析的检测限（10−16m2 量级），熔体气泡度 <68 vol%。这些结果表明，含有 30 vol% 和 50 vol% 具有大接触角的斑晶的硅质岩浆在气泡度变大（至少 >68 vol%）之前具有较低的气体渗透性。这一结果表明，根据对火山气体和天然产物的观测而提出的通过深层火山管道的可渗透流体输送非常缓慢，以至于可能需要其他过程（如剪切变形或岩浆对流）来解释观测结果。
The gas and fluid transport in magmas via permeable flow through interconnected bubble networks controls the rate of outgassing from magmas ascending in volcanic conduits and the fluid transport in the mushy boundary layer of magma reservoirs. Hence, clarifying its mechanism and rate is crucial to understanding the explosivity of volcanic eruptions and the evolution and dynamics of a magma reservoir. Recent experimental studies have determined the gas permeabilities in crystal-free rhyolite and basalt. However, no experimental study has investigated the effect of the crystal contents on the permeable gas transport in magmas. In this study, we performed decompression experiments for hydrous rhyolitic melts having crystallinities of 30 and 50 vol% to examine the effect of crystals on the bubble microstructure and gas permeability during magma vesiculation. Size-controlled (100-meshed) corundum crystals were used as an analog of the phenocrysts in silicic magmas. Microstructural analyses using X-ray CT showed that bubbles coalesce and their connectivity increases with a decrease in the final pressure after the decompression, that is, an increase in the vesicularity. As long as the vesicularities of melt part in the crystal-free basis (melt vesicularity) were similar, no clear effect of the crystallinity on the degree of bubble coalescence and connectivity was observed at melt vesicularities <68 vol%. The corundum showed a large contact angle with aqueous fluid as well as plagioclase and alkaline feldspar; this failed to induce the efficient heterogeneous nucleation and coalescence of bubbles on its surface. The gas permeabilities of all the run products were lower than the detection limits of the present analysis (the order of 10−16m2) at melt vesicularities <68 vol%. These results show that silicic magmas containing 30 and 50 vol% phenocrysts with a large contact angle have low gas permeabilities until the vesicularity becomes large (at least >68 vol%). This result indicates that the permeable fluid transport through a deep volcanic conduit, which has been proposed on the basis of the observations of volcanic gases and natural products, is so slow that other processes, like shear deformation or magma convection, may be needed to explain the observations.