Mixing and the distribution of heterogeneities in a chaotically convecting mantle
Mixing and the distribution of heterogeneities in a chaotically convecting mantle
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混沌对流地幔中的混合和异质性分布
DOI：
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发表时间：
1990
期刊：
影响因子：
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通讯作者：
D. Turcotte
中科院分区：
文献类型：
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作者：
L. Kellogg;D. Turcotte
化学不均匀性是通过大洋岩石圈俯冲，也可能是通过沉积物俯冲和大陆岩石圈拆沉作用引入地幔的。这些不均匀性通过对流混合作用混入地幔。化学性质不同的异质性被对流剪切搅动到周围的地幔基质中，对流剪切使它们变形，增加了异质性和基质之间的接触面积。最后的均匀化是通过扩散来完成的，扩散使厘米级的异质性与基质达到化学平衡。地幔对流的数值模型表明它是混沌的。为了研究混沌流动对混合的影响，我们使用混沌状态下的Lorenz方程作为驱动器，生成了一个充满空间的二维流动。我们指定一个双模式的扩展流函数，导致在一个和两个细胞之间平稳振荡的流动。粒子的路径在时间和空间上都是混沌的。混合非常迅速;初始尺寸为6 km的非均匀体的混合时间为2.4亿年。全地幔对流的混合时间为9.6亿年。洋中脊玄武岩的源区具有“大理石蛋糕”结构;它由经过洋中脊加工、俯冲、拉伸和变薄的物质组成，并且部分或全部均匀化。不完全混合俯冲洋壳在橄榄岩基质中形成榴辉岩条带。这些波段的尺度从6公里到几厘米不等。
Chemical heterogeneities are introduced into the mantle by subduction of oceanic lithosphere and possibly by subduction of sediments and delamination of the continental lithosphere. These heterogeneities are blended into the mantle by convective mixing. Chemically distinct heterogeneities are stirred into the surrounding mantle matrix by convective shear, which deforms them, increasing the area of contact between an heterogeneity and the matrix. Final homogenization is accomplished by diffusion, which brings centimetersized heterogeneities into chemical equilibrium with the matrix. Numerical models of mantle convection suggest that it is chaotic. To study the effect of a chaotic flow on mixing, we have generated a space-filling, two-dimensional flow using the Lorenz equations in the chaotic regime as a driver. We specify a two-mode expansion of the stream function, resulting in a flow which oscillates smoothly between one and two cells. Particle paths are chaotic in time and space. Mixing is very rapid; the mixing time for heterogeneities with an initial size of 6 km is 240 m.y. for layered mantle convection; the corresponding mixing time for whole mantle convection is 960 m.y. The source region for mid-ocean ridge basalts has a “marble cake” structure; it is made up of material which has been processed through ridges, subducted, stretched, and thinned and is either partially or wholly homogenized. The incompletely mixed subducted oceanic crust forms bands of eclogite in the peridotite matrix. These bands range in scale from 6 km to a few centimeters.