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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >The effect of sloped isotherms on melt migration in the shallow mantle: a physical and numerical model based on observations in the Oman ophiolite
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The effect of sloped isotherms on melt migration in the shallow mantle: a physical and numerical model based on observations in the Oman ophiolite

机译：等温线倾斜对浅层地幔中熔体运移的影响：基于阿曼蛇绿岩观测的物理和数值模型

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Field observations in the Oman ophiolite and petrological data are used to constrain a model of melt segregation at the top of the mantle beneath an oceanic spreading centre. Foliations and lineations in outcrops of mantle-derived peridotites oriented at high angle relative to the crust-mantle boundary have been interpreted as the footprint of a former axial asthenospheric convective upwelling several kilometers in cross-section that reached Moho levels. Basaltic melts migrating through this upwelling reacted with their host harzburgites and suffered fractional crystallization. The mantle-crust transition zone at the top of the upwelling is characterized by an very thick (about 400 m) dunite layer whose detailed structure and composition point to the development by compaction of a former "mantle mush". The more important structures (in terms of volume of crystallization products) found in the underlying harzburgites are dunitic-troctolitic horizons a few meters thick and of lateral extent reaching 1 km and more. They crystallized at high temperature (>1190 degreesC) from melts similar to mid-ocean ridge basalts (MORB). These are called "sills" because they are sub-parallel to the crust-mantle boundary, but they can present a moderate dip (15degrees to 20degrees at most) relative to this paleo-horizontal surface. These observations have motivated the modelling of melt segregation by compaction within the crystallization domain inside the top convective boundary layer of the mantle upwelling. Two original inputs to the modelling are considered here: (i) the slope of the iso-curves of melt concentration due to the progressive cooling of the mantle in the boundary layer away from the axis of the rising convective flow; (ii) the reduction in permeability caused by the crystallization of the intergranular melt. Modelling shows that a unique condition is required to generate the troctolite sills and the thick dunite layer nested at the top of the Maqsad diapir: namely a dramatic drop of the interstitial melt concentration at the top of the mantle. Besides, the model developed here allows to scale the time, volume and velocity of the melt segregation. (C) 2004 Elsevier B.V. All rights reserved.
机译：阿曼蛇绿岩的现场观测和岩石学数据被用来约束一个模型，该模型在大洋扩散中心之下的地幔顶部形成了熔体分离模型。相对于地壳-地幔边界成高角度取向的地幔源橄榄岩的露头中的叶状构造和线型已被解释为一个前轴向软流圈对流上升到Moho水平面几公里的足迹。通过该上升流迁移的玄武质熔体与它们的主哈格石反应并发生了部分结晶。上升流顶部的地壳-地壳过渡带的特征是非常厚（约400 m）的榴辉岩层，其详细的结构和组成指向通过压实以前的“地幔糊状物”来发展。在潜在的哈兹伯格岩中发现的更重要的结构（就结晶产物的体积而言）是几米厚的双元-三叠纪层位，横向范围达到1 km或更长。它们在高温（> 1190摄氏度）下从类似于中海脊玄武岩（MORB）的熔体中结晶出来。这些之所以称为“窗台”，是因为它们与地幔的边界不平行，但相对于该古水平面，它们可以呈现出适度的倾角（最多15度到20度）。这些观察结果激发了在地幔上升流的顶部对流边界层内部的结晶域内通过压实作用进行熔体偏析的建模。这里考虑了模型的两个原始输入：（i）由于边界层中地幔逐渐冷却，远离上升的对流流动的轴心，熔体浓度的等曲线的斜率; （ii）由晶间熔体的结晶引起的渗透率降低。模拟表明，需要产生独特的条件，才能生成斜方晶石基岩和嵌套在Maqsad diapir顶部的厚厚的榴辉岩层：即在地幔顶部的间隙熔体浓度急剧下降。此外，这里开发的模型允许缩放熔体偏析的时间，体积和速度。 （C）2004 Elsevier B.V.保留所有权利。

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