Chemically driven growth and resorption of bubbles in a multivolatile magmaticsystem
Chemically driven growth and resorption of bubbles in a multivolatile magmaticsystem
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多挥发性岩浆系统中化学驱动的气泡生长和再吸收
DOI：
10.1016/j.chemgeo.2010.05.010
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发表时间：
2010
期刊：
影响因子：
3.9
通讯作者：
M. Nakamura
中科院分区：
文献类型：
--
作者：
Yoshimura;S.;M. Nakamura
来自深层的富含二氧化碳的流体在富含水的浅层岩浆中流动，并与熔体发生挥发性交换。我们对 H2O-CO2 系统中的无气泡和含气泡流纹岩熔体进行了水热实验，以探索流体和岩浆之间化学再平衡的动力学。在无气泡实验中，首先在800℃和100MPa下合成水饱和的流纹岩熔体，并在相同的PT条件下与富含二氧化碳的流体相互作用3小时。在挥发性交换时，熔体暂时变得不饱和，因为水迅速从熔体中扩散出来，而二氧化碳的溶解速度要慢得多。在含气泡实验中，流纹岩熔体在800℃和100MPa下用H2O-CO2流体饱和，减压至50MPa，然后保持5.56h。减压时会产生富含水的气泡，最初会长大，但随后在停留时间内溶解在熔体中。这种气泡再吸收归因于熔体在与富含二氧化碳的周围流体重新平衡时水分耗尽。这种水分消耗和由此产生的气泡再吸收是多挥发性系统所特有的，例如H2O-CO2，其中最终组分的扩散率和溶解度不同。这种机制中发生的气泡再吸收可以解释富含二氧化碳、缺乏气泡的黑曜石火山碎屑的成因，这些火山碎屑被认为经历了富含二氧化碳的流体流动。我们还讨论了这种效应对岩浆密度的可能影响。少量二氧化碳的溶解可能会导致从含水熔体中排出大量的水，这可能会增加流体分数，从而使流纹岩熔体在 100 和 200MPa 下的堆积密度分别降低高达 188 和 104kg/m3。我们认为这样的过程有可能引发火山喷发。
A carbon dioxide-rich fluid from a deep source fluxes in a water-rich shallow-stored magma and causes a volatile exchange with the melt. We carried out hydrothermal experiments on both bubble-free and -bearing rhyolitic melts in an H2O–CO2system to explore the kinetics of chemical re-equilibration between the fluid and the magma. In the bubble-free experiment, a water-saturated rhyolitic melt was first synthesized at 800°C and 100MPa, and this interacted with a carbon dioxide-rich fluid under the same PT conditions for a period of 3h. The melt temporarily became undersaturated upon the volatile exchange, as water quickly diffused out of the melt, while dissolution of the carbon dioxide was much slower. In the bubble-bearing experiments, the rhyolitic melt was saturated with H2O–CO2fluids at 800°C and 100MPa, decompressed to 50MPa, and then kept for up to 5.56h. Water-rich bubbles were generated by decompression, and initially grew, but then dissolved in the melt during the retention time. This bubble resorption was attributed to water depletion of the melt upon re-equilibration with the carbon dioxide-rich surrounding fluid. Such water depletion and resulting bubble resorption are specific to multivolatile systems, such as H2O–CO2, in which the diffusivity and solubility differ among the end components. The bubble resorption occurring in this mechanism may explain the genesis of carbon dioxide-rich, bubble-poor obsidian pyroclasts, which are considered to have undergone carbon dioxide-rich fluid fluxing. We also discuss possible consequences of this effect on the bulk magma density. The dissolution of a small amount of carbon dioxide may cause expulsion of a much larger amount of water from the hydrous melt, which may increase fluid fraction and thus decrease the bulk density by up to 188 and 104kg/m3at 100 and 200MPa, respectively, for rhyolitic melts. We suggest that such a process has the potential to trigger a volcanic eruption.