A Laboratory Investigation of Assimilation at the Top of a Basaltic Magma Chamber
A Laboratory Investigation of Assimilation at the Top of a Basaltic Magma Chamber
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玄武岩岩浆室顶部同化的实验室研究
DOI：
10.1086/629117
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发表时间：
1987
期刊：
影响因子：
--
通讯作者：
J. S. Turner
中科院分区：
文献类型：
--
作者：
I. Campbell;J. S. Turner
通过一系列室内模拟实验，研究了影响岩浆房顶板同化机制和同化量的流体动力学过程。首先在实验室储罐的顶部冷却和结晶含有Na2CO3的水溶液，形成规定组成的固体屋顶层。然后，将罐中的水排出，并重新注入更热的溶液，即$$Na_{2}CO_{3}$$(成分相同或不同)，或者在两个实验中重新注入$$KNO_{3}$$。在所有情况下，金属顶都保持在$$Na_2}CO_{3}-H_{2}O$$体系的共晶温度以下。实验继续进行，直到实现了实质性的结晶，在某些情况下还实现了完全凝固。从致密的Na2CO310H2O构成的固体屋顶层开始，在共晶的Na2CO3一侧结冰，再从下面较热的低共晶溶液开始，液层迅速对流冷却，伴随着屋顶的熔化，产生成分稠密的液体，并向下对流。当顶板为共晶组成的固体，且热输入流体密度较大时，形成光稳液层。这两个液层由一个双扩散界面隔开，热量通过该界面快速传递，但几乎没有传质。随着下层在底部冷却结晶，其Na2CO3浓度降低，结晶释放的潜热通过界面向上输送，贡献了顶板熔化所需的热量。在屋顶和输入溶液的平均成分相当的实验中，熔化还导致了稳定的上层液层的发展，因为较致密的$Na2CO3}10H2O$$晶体脱落，留下较轻的共晶成分熔体。对于不同密度的溶质(KNO_3)，该体系最初的行为类似于Na_2CO_3实验，并产生了一个轻质的屋顶层，但后来两个液层的密度由于冷却和结晶而演变，直到它们变得相等，从而导致颠覆和充分混合。玄武岩岩浆室的含义是，熔融的物质将留在岩浆室的顶部，并在化学上与底部的玄武岩岩浆隔离。在玄武岩岩浆室中，顶板岩石熔融产生的岩浆比下面的岩浆轻。AFC(分步结晶同化)过程意味着同化所需的热量来自结晶潜热，这两个过程同时发生在混合良好的岩浆中。释放出的熔体形成了一层轻薄的屋顶层，最初可能会与下面的一些玄武岩岩浆混合，但后来混合得很少。其结果是，同化在空间和时间上与结晶分离。上层和下层之间的界面仍然允许垂直快速的热传输，因此地板上的结晶提供热量来吸收屋顶，但在结晶时，很少有屋顶材料被结合到下层。在后来的阶段，被污染的岩浆在屋顶结晶，因此分离结晶遵循同化作用，而不是同时发生在同一岩浆体内的两个过程。真正的AFC仅限于较低的玄武岩层，但受到可以通过将玄武岩层与熔融的屋顶层分开的双重扩散界面传输的材料数量的限制。如果屋顶的大量积木穿过上层并在下层融化，AFC确实变得重要。
The fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments. An aqueous solution of $$Na_{2}CO_{3}$$ was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition. The tank was then drained and refilled with warmer solution, either $$Na_{2}CO_{3}$$ (of the same or different composition) or, in two experiments, $$KNO_{3}$$. The metal top in all cases was held below the eutectic temperature for the system $$Na_{2}CO_{3}-H_{2}O$$. The experiments were continued until substantial crystallization, and in some cases complete solidification, were achieved. Starting with a dense solid roof layer of $$Na_{2}CO_{3}10H_{2}O$$ and ice on the $$Na_{2}CO_{3}$$ side of the eutectic, and a warmer eutectic $$Na_{2}CO_{3}$$ solution below, there was rapid convective cooling of the liquid layer accompanied by melting of the roof, producing a compositionally dense liquid which convected downwards. When the roof was a solid of eutectic composition and the warm input fluid was much denser, a light stable liquid layer formed. The two liquid layers were separated by a double-diffusive interface through which heat was transported rapidly, but through which there was little mass transfer. As the lower layer cooled and crystallized at the bottom, its $$Na_{2}CO_{3}$$ concentration decreased, and the latent heat released by crystallization was transported upwards through the interface and contributed to the heat required for melting at the roof. In an experiment in which the mean compositions of the roof and the input solution were comparable, melting also led to the development of a stable upper liquid layer, as denser $$Na_{2}CO_{3}10H_{2}O$$ crystals fell off, leaving a lighter melt of eutectic composition behind. With a different denser solute ($$KNO_{3}$$) below, the system at first behaved similarly to the $$Na_{2}CO_{3}$$ experiment and produced a light roof layer, but later the densities of the two liquid layers evolved due to cooling and crystallization until they became equal, resulting in overturning and thorough mixing. The implication for a basaltic magma chamber, in which the magma produced by melting of the roof rocks is lighter than the magma below, is that the melted material will remain at the top of the chamber and be chemically isolated from the basaltic magma at the bottom. The process of AFC (assimilation with fractional crystallization) implies that the heat required for assimilation comes from the latent heat of crystallization, and that these two processes occur simultaneously in a single body of well-mixed magma. The melt released forms a light roof layer which may initially mix with some of the underlying basaltic magma, but then mixes very little. As a consequence, assimilation is separated from the crystallization in space and time. The interface between the upper and lower layers still allows rapid thermal transport vertically, so that crystallization at the floor supplies heat to assimilate the roof, but little roof material is incorporated in the lower layer as it crystallizes. At a later stage the contaminated magma at the roof crystallizes so that fractional crystallization follows assimilation, rather than the two processes occurring simultaneously from the same body of magma. True AFC is confined to the lower basaltic layer but is limited by the amount of material which can be transported across the double-diffusive interface which divides the basaltic layer from the melted roof layer. AFC does become important if large numbers of blocks of the roof fall through the upper layer and melt in the lower layer.