Influence of eruptive style on volcanic gas emission chemistry and temperature
Influence of eruptive style on volcanic gas emission chemistry and temperature
复制标题
喷发方式对火山气体喷发化学和温度的影响
作者：
C. Oppenheimer;B. Scaillet;A. Woods;A. J. Sutton;T. Elias;Y. Moussallam;Y. Moussallam
当岩浆在地壳中上升并溶解挥发物时，气泡形成。这些气泡随着岩浆减压和结晶的进行而发生化学和物理上的变化。一般认为气体与熔体保持热平衡，但气体和熔体氧化还原状态之间的关系仍有争议。在这里，使用吸收光谱，我们报告的组成，从熔岩湖的Kallauea火山，夏威夷，并计算气体排放的平衡条件。我们的观察跨越之间的过渡更有力和更少的脱气制度。它们揭示了高达250 °C的温度范围，以及相对于固体岩石缓冲层，随着气体温度的降低，气体逐渐氧化。我们认为，这些现象是改变气泡尺寸的结果。我们发现，即使是更粘稠的岩浆，快速上升的气泡可以冷却冷却，并失去其相关的熔体的氧化还原签名。该过程可导致氧化还原敏感性气体物质的丰度的快速变化。在许多火山监测气体成分，以支持危险评估，但观测的时间平均值可能掩盖脱气动态引起的这种变化。此外，所观察到的气体和熔体之间的氧化还原脱钩呼吁谨慎使用熔岩化学推断相关的火山气体的组成。根据对夏威夷的卡拉韦厄火山熔岩湖的气体排放的观察，在岩浆上升过程中，火山气体和熔体的氧化还原状态可以变得脱钩。快速上升的气泡的冷却改变了氧化还原敏感气体物种的丰度。
Gas bubbles form as magmas ascend in the crust and exsolve volatiles. These bubbles evolve chemically and physically as magma decompression and crystallization proceed. It is generally assumed that the gas remains in thermal equilibrium with the melt but the relationship between gas and melt redox state is debated. Here, using absorption spectroscopy, we report the composition of gases emitted from the lava lake of Kīlauea Volcano, Hawaii, and calculate equilibrium conditions for the gas emissions. Our observations span a transition between more and less vigorous-degassing regimes. They reveal a temperature range of up to 250 °C, and progressive oxidation of the gas, relative to solid rock buffers, with decreasing gas temperature. We suggest that these phenomena are the result of changing gas bubble size. We find that even for more viscous magmas, fast-rising bubbles can cool adiabatically, and lose the redox signature of their associated melts. This process can result in rapid changes in the abundances of redox-sensitive gas species. Gas composition is monitored at many volcanoes in support of hazard assessment but time averaging of observations can mask such variability arising from the dynamics of degassing. In addition, the observed redox decoupling between gas and melt calls for caution in using lava chemistry to infer the composition of associated volcanic gases.The redox state of volcanic gases and melts can become decoupled during magma ascent, according to observations of gas emissions from Kīlauea’s lava lake, Hawaii. Cooling of fast-rising bubbles changes the abundance of redox-sensitive gas species.