The impact of pre-existing gas on the ascent of explosively erupted magma
The impact of pre-existing gas on the ascent of explosively erupted magma
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预先存在的气体对爆发性喷发岩浆上升的影响
DOI：
10.1007/s00445-009-0276-8
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发表时间：
2009
影响因子：
3.5
通讯作者：
J. Gardner
中科院分区：
文献类型：
--
作者：
J. Gardner
大多数（如果不是全部）岩浆在喷发前都在深处含有气泡。 Those bubbles play a crucial role in eruption dynamics, by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface.然而，这种加速必须有一个限度，因为气泡不可能无限快地增长。为了探索这个极限，进行了一系列实验以确定气泡高硅流纹岩可以减压的最大速率。流纹岩熔体在 150 MPa 下水合，含有约 5.3 wt.% 的溶解水并含有 7 至 18 vol.% 的气泡，当从 150 MPa 至 78 MPa 以高达 1.2 MPa s−1 的速率减压时，可以在 875°C 下达到平衡脱气，当进一步减压至 42 MPa 时，脱气速率高达 1.8 MPa s−1。相比之下，如果减压速度快于 0.015–0.025 MPa s−1，同一流纹岩无法在 750°C 下达到平衡脱气。与其他已发表的实验相结合，发现温度每升高 50-75°C，平衡脱气的最大减压速率就会增加十倍。与假设平衡脱气的管道流模型的预测相比，我们发现此类模型大大高估了相对较冷的流纹岩可以减压的速率，而该假设对于热流纹岩来说在很大程度上是正确的，因此对于大多数其他岩浆而言，所有这些岩浆的粘度都比流纹岩低。此外，大多数在高压下尺寸为 20-30 µm 的气泡在低压下从气泡中消失。这种缺失表明，只有在火山浮石中看到的相对较大的囊泡可能是喷发前气泡的遗迹，即使小气泡最初存在于深处。
Most, if not all, magmas contain gas bubbles at depth before they erupt. Those bubbles play a crucial role in eruption dynamics, by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface. There must be a limit to that acceleration, however, because gas bubbles cannot grow infinitely fast. To explore that limit, a series of experiments was undertaken to determine the maximum rate at which bubbly high-silica rhyolite can decompress. Rhyolite melt that was hydrated at 150 MPa with ~5.3 wt.% dissolved water and contained 7 to 18 vol.% bubbles can degas in equilibrium at 875°C when decompressed at rates up to 1.2 MPa s−1 from 150 to 78 MPa, and up to 1.8 MPa s−1 when decompressed further to 42 MPa. In contrast, that same rhyolite cannot degas in equilibrium at 750°C if decompressed faster than 0.015–0.025 MPa s−1. When combined with other published experiments, the maximum rate of decompression for equilibrium degassing is found to increase by a factor of ten for every 50–75°C increase in temperature. When compared to predictions from conduit flow models that assume equilibrium degassing, it is found that such models greatly over-estimate the rate at which relatively cold rhyolite can decompress, whereas that assumption is largely correct for hot rhyolite, and thus for most other magmas, all of which are less viscous than rhyolite. In addition, most bubbles that were 20–30 µm in size at high pressure were lost from the population at low pressure. That absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre-eruptive bubbles, even if small bubbles were originally present at depth.