Carbon dioxide transport in crustal magmatic systems
Carbon dioxide transport in crustal magmatic systems
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DOI：
10.1016/j.epsl.2011.05.039
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发表时间：
2011-07
影响因子：
5.3
通讯作者：
S. Yoshimura;M. Nakamura
中科院分区：
文献类型：
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作者：
S. Yoshimura;M. Nakamura
年轻火山玻璃的挥发性化学物质表明，浅埋的地壳岩浆经常遭受来自下面的富含二氧化碳的流体的开放系统添加，可能来自地幔玄武岩源（“二氧化碳通量”）。然而，这种流体输送的实际机制知之甚少。为了限制挥发物的传输机制，我们将这种现象表述为反应性传输过程，并阐明了系统中化学交换的基本特征。该模型假设，一个富含二氧化碳的流体被引入到一个富含水的流纹岩岩浆柱从下面和上升以恒定的速度，而挥发性的交换发生在流体和熔体之间。研究了两种交换模式。一种是平衡模式，其中挥发物交换在所有柱深度处瞬时实现。第二种是扩散模式，其中挥发物交换速率受扩散限制。在平衡模式中，整个柱的再平衡程度仅由整合流体与熔体的质量比控制。在扩散模式中，再平衡的程度由Damköhler数控制，Damköhler数是代表平流时间与扩散时间之比的无量纲参数。当二氧化碳的达姆克勒数超过10时，扩散交换与平衡交换就无法区分了。这两种交换模式产生的二氧化碳和水在熔体中的浓度之间的负相关性，这不能解释没有显着的结晶传统的脱气模型。作为火山气体从柱中喷出的流体将其成分从富含二氧化碳变为富含水，并且在流动过程中喷出速率单调下降。模拟使我们能够约束流体传输的机制。对于毕晓普凝灰岩的熔体包裹体数据（Wallace等人，1999;安德森等人，2000年），岩浆中的流体速度估计为10− 6-10− 7米/秒。流体输送的相应机制可能包括渗透率约为10− 15 m2的渗透流或半径为4-7 mm的单个气泡的浮力上升。
The volatile chemistry of juvenile volcanic glasses has suggested that shallow-stored crustal magmas often suffer the open-system addition of a carbon-dioxide-rich fluid from below, probably from a mantle-derived basaltic source (“carbon dioxide fluxing”). However, the actual mechanism of such a fluid transport is poorly understood. To constrain the volatile transport mechanism, we formulated this phenomenon as a reactive transport process and clarified the fundamental characteristics of chemical exchange in the system. The model assumes that a carbon-dioxide-rich fluid is introduced into a water-rich rhyolitic magma column from below and ascends at a constant velocity whilst a volatile exchange takes place between the fluid and melt. Two types of exchange modes were examined. One is the equilibrium mode where the volatile exchange is instantaneously achieved at all column depths. The second is the diffusive mode where the volatile exchange is rate limited by diffusion. In the equilibrium mode, the extent of re-equilibration of the entire column is controlled solely by the mass ratio of the integrated fluid to the melt. In the diffusive mode, the extent of re-equilibration is controlled by the Damköhler number, a dimensionless parameter representing the ratio of the advection time to the diffusion time. When the Damköhler number for carbon dioxide exceeds 10, the diffusive exchange becomes indistinguishable from the equilibrium exchange. Both exchange modes produce a negative correlation between the concentrations of carbon dioxide and water in the melt, which cannot be explained by conventional degassing models without significant crystallisation. The fluid emitted from the column as a volcanic gas changes its composition from carbon dioxide rich to water rich, and the emission rate decreases monotonically during fluxing. The simulation enables us to constrain the mechanism of fluid transport. For the melt inclusion data from the Bishop Tuff (Wallace et al., 1999; Anderson et al., 2000), fluid velocity in this magma was estimated to be 10− 6–10− 7m/s. The corresponding mechanism of fluid transport may include permeable flow with a permeability of ~ 10− 15m2or a buoyant ascent of individual bubbles with a radius of 4–7 mm.