Dynamical and chemical effects of melting a heterogeneous source
Dynamical and chemical effects of melting a heterogeneous source
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熔化异质源的动力学和化学效应
DOI：
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发表时间：
1989
期刊：
影响因子：
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通讯作者：
S. Daly
中科院分区：
文献类型：
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作者：
F. Richter;S. Daly
来自地幔的熔体在微量元素和同位素比率上的地球化学变异性要求化学上的不均匀来源。这就需要了解来自不同来源的熔体分离的动力学以及化学分馏和输送的相关影响。一系列理想化的数值模型问题，基于部分熔体源区的两相流动表示，被用来处理局部非均质的物理和地球化学性质如何对整体分凝熔体做出贡献。我们发现，当非均质表示比背景贫化地幔更容易熔化的物质时，它们将产生高熔体分数的离散行波脉冲，可以用二维孤立波来理解，我们分别说明了这些孤立波是所用控制方程的解。非均质熔化产生的孤立波状脉冲取决于熔化反常的大小，与部分熔化系统的自然压实长度相比，当过剩熔化区域较大时，孤波脉冲最有效。在孤立波的地球化学输运方面，我们发现它们留下了与产生孤立波的快速熔融区相关的任何独特的微量元素浓度，因此物理和化学效应是解耦的。孤立波对不相容的痕量元素的化学输运仍然是反常的，在微量元素浓度变得与低熔体分数背景下没有什么不同的情况下，局部有更多的富集液(熔体)以大的相速度运动。一般的结论是，当非均相源部分熔化时，它可以产生快速移动的熔体分数脉冲或孤立波，但这些脉冲或孤立波并不带有产生它们的局部非均质性的任何化学特征。这表明，地幔熔体的地球化学性质应该用与部分熔融区相当的长度尺度上的平均值来解释，而不是特别受到局部异常成分的偏向，即使这些成分是在上升流过程中优先熔融的。
The geochemical variability of mantle-derived melts in both trace element and isotope ratios is such as to require a chemically heterogeneous source. This raises the need to understand the dynamics of melt segregation from a heterogeneous source and the associated effects of chemical fractionation and transport. A series of idealized numerical model problems, based on a two-phase flow representation of a partially molten source region, are used to address how the physical and geochemical properties of local heterogeneities contribute to the overall segregating melts. We find that when the heterogeneities represent material that melts more easily than the background depleted mantle they will give rise to discrete traveling pulses of high melt fraction that can be understood in terms of two-dimensional solitary waves, which we separately show are solutions to the governing equations used. The generation of solitary wave-like pulses by heterogeneous melting depends on the scale of the melting anomaly compared to the natural compaction length of the partially molten system, being most effective when the region of excess melting is large. In terms of the geochemical transport by solitary waves we find that they leave behind any distinctive trace element concentration associated with the region of more rapid melting that produces the wave and therefore that the physical and chemical effects decouple. The chemical transport of incompatible trace elements by solitary waves is still anomalous in the sense that while the trace element concentration becomes no different than that of the lower melt fraction background there is locally much more of the enriched fluid (melt) moving with a large phase velocity. The general conclusion is that when a heterogeneous source partially melts it can give rise to fast moving melt fraction pulses or solitary waves, but these do not carry with them any chemical distinctiveness of the local heterogeneities that produce them. This suggests that the geochemical properties of mantle-derived melts should be interpreted in terms of averages over length scales comparable to the region of partial melting, and not particularly biased by local anomalous components even when these are preferentially melted during upwelling.