Lead isotope variability in olivine-hosted melt inclusions from Iceland
Lead isotope variability in olivine-hosted melt inclusions from Iceland
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DOI：
10.1016/j.gca.2008.05.034
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发表时间：
2008-08
影响因子：
5
通讯作者：
J. Maclennan
中科院分区：
文献类型：
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作者：
J. Maclennan
冰岛雷克雅内斯半岛原始熔岩流中一套橄榄石熔融包裹体的铅同位素和微量元素组成显示出极大的变异性。大部分的这种变化是存在于从Háleyjabunga，一个耗尽的苦橄岩熔岩盾，爆发13 ka.208Pb/206 Pb组合物在这个样本跨度50-90%的总范围内发现在大西洋MORB的手标本的内含物的组成，表明高振幅的组成异质性是存在于地幔源的熔体聚集，形成一个单一的喷发。熔融包裹体中的微量元素和同位素趋势与Reykjanes半岛年轻熔岩流的全岩样品中的趋势一致，并将总的变化范围扩大到更亏损的成分。包裹体中不相容的微量元素和铅同位素组成具有强烈的耦合性，接近于极端熔融包裹体组成之间的二元混合趋势。这些关系表明，熔融包裹体中微量元素的变化反映了进入雷克雅未克半岛下熔融区的地幔源区成分的不均匀性。大的正Sr浓度异常存在于三个包裹体，但不相关的地幔熔融或源的变化的指标，并可能出现在地壳存储过程中与斜长石反应。非均质地幔的部分熔融被预测为产生具有广泛组成的熔体，填充在微量元素-同位素空间中的大体积。然而，在熔体包裹体中观察到的成分变化接近二元混合曲线。这些观察结果可以通过熔体混合的两阶段模型来解释。第一阶段发生在地幔中的多孔通道中，并在包裹体捕获之前发生。这一混合阶段产生了双峰分布的熔体成分，是从通道供应到莫霍面和下地壳岩浆透镜体。第二阶段的混合发生在这些腔室中，产生的二元混合趋势记录在夹杂物组合物。因此，在雷克雅未克半岛的熔融包裹体和全岩样品中观察到的同位素组成的分布是由熔融混合控制的。这些结果具有重要的意义的玄武岩成分的解释在不同的组成实体内的上涌固体地幔洋中脊和海洋岛屿。
The lead isotope and trace element compositions of a suite of olivine-hosted melt inclusions in primitive lava flows from the Reykjanes Peninsula in Iceland show extreme variability. Much of this variability is present in the composition of inclusions from one hand specimen of Háleyjabunga, a depleted picrite lava shield that erupted 13 ka.208Pb/206Pb compositions in this sample span 50–90% of the total range found in Atlantic MORB, indicating that high-amplitude compositional heterogeneity is present in the mantle source of melts that aggregated to form a single eruption. The trace element and isotopic trends in the melt inclusions are coincident with those in whole rock samples from young lava flows of the Reykjanes Peninsula, and extend the total range of variation towards more depleted compositions. The incompatible trace element and lead isotope compositions of the inclusions are strongly coupled and lie close to binary mixing trends between the extreme melt inclusion compositions. These relationships indicate that the trace element variation in the melt inclusions reflects heterogeneity in the composition of the mantle source entering the melting region under the Reykjanes Peninsula. Large positive Sr concentration anomalies are present in three of the inclusions, but do not correlate with indicators of mantle melting or source variations and are likely to arise by reaction with plagioclase during crustal storage. Fractional melting of heterogeneous mantle is predicted to generate melts with a wide range of compositions, filling a large volume in trace element–isotope space. However, the compositional variations observed in the melt inclusions lie close to binary mixing curves. These observations may be accounted for by a two-stage model of melt mixing. The first stage occurs in porous channels that transport melt in the mantle and takes place before inclusion entrapment. This mixing stage generates a bimodal distribution of melt compositions that is supplied from the channels to sub-Moho and lower crustal magma lenses. The second stage of mixing occurs in these chambers, producing the binary mixing trends recorded in the inclusion compositions. The distribution of isotopic compositions observed in the melt inclusions and whole rock samples from the Reykjanes Peninsula is therefore controlled by melt mixing. These results have important implications for the interpretation of basalt composition in terms of distinct compositional entities within the upwelling solid mantle under mid-ocean ridges and ocean islands.