Crystallization in a double‐diffusive system
Crystallization in a double‐diffusive system
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双扩散系统中的结晶
DOI：
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发表时间：
1980
期刊：
影响因子：
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通讯作者：
J. S. Turner
中科院分区：
文献类型：
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作者：
C. F. Chen;J. S. Turner
这里报道的实验是第一次尝试在一个简单的实验室系统中模拟岩浆房结晶过程中发生的一些物理过程，并研究不同分子扩散速率对层状火成岩形成的可能影响。我们的实验室研究结果表明，分层的熔体和“双扩散”的岩浆凝固过程中的影响时，应考虑到发展理论的分层。解释，我们已经扩展了早期的实验双扩散对流，这表明如何层可以很容易地形成平滑梯度时，两个组件具有不同的分子扩散率的密度梯度上有相反的效果。新的实验包括结晶的影响，并使用了在各种配置中冷却的Na2CO3水溶液。当冷却是从顶部，晶体形成水平层，无论初始流体是均匀的，分层与恒定的梯度，或分层在几个不同的层。主要的影响是形成一个冷的，但轻的流体层对冷却的上边界的密度晶体的增长。晶体在上层（正在剧烈转化）中较小且紧密堆积，而在较低、较静止的流体中较大且较松散堆积。在有预先存在的层的情况下，当晶体穿过界面时，晶体的生长速率突然增加。在侧壁冷却和恒定梯度的情况下，结晶后留下的较轻的溶液在薄的边界层中向上流动，直到罐的顶部。外部流动由几乎水平的转换层中的环流组成，当轻质流体在顶部聚集时，每一层都缓慢地降低。当恒定梯度的Na2CO3从下面冷却时，结晶产生“手指”不稳定性，导致上面的流体中混合层的生长。使用相反梯度的Na2CO3和K2CO3（顶部富含Na2CO3）的实验表明，晶体沉降到它们自己的密度水平，然后再溶解可以逆转梯度的原始意义，并在液体中产生小规模的双扩散分层。
The experiments reported here are a first attempt to model in a simple laboratory system some of the physical processes occurring during crystallization in magma chambers and to examine possible effects of different molecular diffusion rates on the formation of layered igneous rocks. Our laboratory results suggest that stratification in the melt and ‘double-diffusive’effects during solidification of the magma should be taken into account when developing theories of the layering. Explicitly, we have extended earlier experiments on double-diffusive convection, which show how layers may readily form from smooth gradients when two components with different molecular diffusivities have opposing effects on the density gradient. The new experiments included the influence of crystallization and have used an aqueous solution Of Na2CO3 cooled in various configurations. When cooling was from the top, the crystals formed in horizontal layers, whether the initial fluid was homogeneous, was stratified with a constant gradient, or was stratified in several distinct layers. The dominant effect was the formation of a cold but light fluid layer against the cooled upper boundary as the denser crystals grew. The crystals were small and closely packed in the upper layer (which was vigorously converting) and larger and more loosely packed in the lower, more quiescent fluid. In the case where there were preexisting layers there was an abrupt increase in the growth rate of crystals as they crossed an interface. With sidewall cooling and a constant gradient the lighter solution left behind after crystallization streamed upward in a thin boundary layer right to the top of the tank. The outer flow consisted of circulation in nearly horizontal converting layers, each of which was depressed slowly as the light fluid collected at the top. When a constant gradient of Na2CO3 was cooled from below, crystallization produced a ‘finger’ instability, leading to the growth of a mixed layer in the fluid above. An experiment using opposing gradients of Na2CO3 and K2CO3 (with Na2CO3 rich on top) showed that crystals settling to their own density level and then redissolving can reverse the original sense of the gradient and produce small-scale double-diffusive layering in the liquid.