Fingerprint of silicic magma degassing visualised through chlorine microscopy
Fingerprint of silicic magma degassing visualised through chlorine microscopy
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通过氯显微镜观察硅质岩浆脱气的指纹
DOI：
10.1038/s41598-018-37374-0
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发表时间：
2019
影响因子：
4.6
通讯作者：
M.
中科院分区：
文献类型：
--
作者：
Yoshimura;S.;Kuritani;T.;Matsumoto;A.;Nakagawa;M.
富含挥发分的硅质岩浆要么以火山碎屑和高温气体的混合物的形式爆炸性喷发，要么以非爆炸性的方式喷出熔岩。喷发方式的分叉被广泛认为是由上升过程中气泡岩浆的开放系统气体损失的效率控制的。然而，气体如何从高粘性岩浆中逃逸的根本问题仍然没有解决，因为气体流动的路径很少被保存在致密的熔岩中。在这里，我们展示了这样的路径在磨砂玻璃中可视化，使用高分辨率的氯(Cl)映射分析，对日本幕凯山火山的流纹岩熔岩进行分析。结果表明，该玻璃的氯离子含量高度不均匀。玻璃中氯含量的空间分布表明，挥发物向大多数气泡扩散，但气泡向致密熔体塌陷，而不是生长。所有的观察，结合熔体包裹体分析，都表明在上升过程中，在几天到两周的时间里，泡化、相互连接的气泡通道的形成、通过通道的开放系统气体损失和通道坍塌重复发生。这个循环在厘米大小的岩浆中以不同的时间重复。
Volatile-rich silicic magma erupts either explosively as a jet of a mixture of pyroclasts and high-temperature gas, or non-explosively to effuse lava. The bifurcation of the eruption style is widely recognised as being controlled by the efficiency of open-system gas loss from vesiculated magma during ascent. However, the fundamental question of how the gas escapes from highly viscous magma still remains unsolved because the pathways of gas flow are rarely preserved in dense lava. Here we show that such pathways are visualised in groundmass glass using high-resolution chlorine (Cl) mapping analysis on the rhyolitic lava of the Mukaiyama volcano, Japan. The results showed that the glass was highly heterogeneous in Cl content. A spatial distribution of the Cl content in the groundmass glass showed that volatiles diffused towards most bubbles, but the bubbles collapsed into the dense melt rather than growing. All observations, in combination with melt inclusion analysis, indicate that vesiculation, the formation of interconnected bubble channels, open-system gas loss via the channels, and channel collapse repeated within the period of a few days to two weeks during ascent. This cycle repeated individually in centimetre-sized portions of magma with different timing.