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首页> 外文期刊>Contributions to Mineralogy and Petrology >Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle
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Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

机译：渗碳橄榄岩的熔体变异性：一项实验研究，用于上地幔中玄武岩的反应性迁移

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Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.
机译：上地幔中的熔岩反应记录在各种超镁铁质岩石中，并且是从源区到地表的过程中改变熔体成分的重要过程。这项实验研究评估了在最高地幔条件下，与玄武岩反应后，玄武岩玄武岩的组成变异性。渗透反应过程是通过三层结构模拟的：原始的玄武岩粉末（“熔体层”）被“橄榄岩层”和一层玻璃碳球（“熔体陷阱”）覆盖。来自熔体层的熔体被迫穿过橄榄岩层移动到熔池中。实验在1,170-1,290°C的温度范围内以0.65和0.8 GPa进行。在此PT范围内，代表在海洋扩散中心下方的软流层和岩石圈之间的过渡带（热边界层）中遇到的条件，熔体受到分馏，橄榄岩部分熔融（T s 〜1,260°C）。在每个实验装料中研究了熔体与橄榄石之间反应对熔体组成的影响。橄榄岩层中的淬火熔体比熔体层玻璃具有更大的成分变化。通过降低熔体层中不相容元素的含量和降低熔体相中扩散扩散的平衡，降低温度时，熔体和橄榄岩层中玻璃之间的差异变得更加重要。在1,290°C，辉石的优先溶解会使熔融液富集在二氧化硅中，并稀释成不相容的元素。此外，由于尖晶石的溶解，液体在较高温度下变得越来越富集Cr 2 O 3 。液体中的二氧化硅含量在1,260°C时下降，而不相容的元素由于结晶水平的提高而开始在熔体中富集。在研究的最低温度下，增加的碱含量会导致二氧化硅由于反应性分馏而增加。渗透性玄武岩通过上地幔热边界层的普遍渗入可以在MORB上施加高Si“低压”特征。这可以解释浅斜斜橄榄石橄榄石中opx + p的富集和橄榄石辉长岩的形成。

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