NbTa-rich mantle amphiboles and micas: Implications for subduction-related metasomatic trace element fractionations
NbTa-rich mantle amphiboles and micas: Implications for subduction-related metasomatic trace element fractionations
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DOI：
10.1016/0012-821x(95)00037-d
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发表时间：
1995-04
影响因子：
5.3
通讯作者：
D. Ionov;A. Hofmann
中科院分区：
文献类型：
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作者：
D. Ionov;A. Hofmann
我们报告了从碱性玄武岩中的地幔捕虏体中分离出来的角闪石和云母的微量元素组成，并通过 ICP-MS 进行了分析。相对于原始地幔成分，脉状角闪石和云母中 Nb 和 Ta 高度富集（50-200 倍），而 Th 和 U 则贫化。一些浸染状角闪石没有如此极端的NbTa富集，但角闪石和单斜辉石之间的NbTa分配系数非常高，范围为10至85。与天然地幔角闪石的结果形成明显对比，最近报道的角闪石和熔体之间的Nb和Ta分配系数非常低[1,2]。造成明显矛盾的原因可能在于角闪石的组成或流体相（富含二氧化硅的水性流体而不是硅酸盐或碳酸盐熔体）。无论哪种情况，我们的观察都表明，角闪石和云母可能是 Nb 和 Ta 的重要宿主，在确定俯冲相关火山岩几乎普遍相对 NbTa 贫化的根本原因时，不能忽视它们。我们提出了一个交代模型，用于创建相对于 Th、U 和轻稀土元素而言 Nb 和 Ta 匮乏的源区。俯冲板片脱水产生的流体通过上覆地幔楔上升并沉淀角闪石。包括 Nb 和 Ta 在内的高度不相容元素随流体转移到楔形物中，角闪石的“开放系统”沉淀将微量元素分馏，从而在残余流体中产生较低的 (Nb,Ta)/(Th,U,LREE) 比率。当这种流体进一步移动时，它要么直接引起楔形热区域的部分熔化，要么通过宿主橄榄岩中浸染角闪石的“封闭系统”结晶而被消耗，随后可以经历部分熔化。无论哪种情况，与俯冲相关的岩浆源区都富含高度不相容的微量元素，但不富含铌和钽。该模型可以被视为解释弧岩浆化学的已发表模型的补充或替代。
We report the trace element compositions of amphibole and mica separated from mantle xenoliths in alkali basalts and analyzed by ICP-MS. Nb and Ta are highly (50–200-fold) enriched in vein amphibole and mica relative to primitive mantle compositions, whereas Th and U are depleted. Some disseminated amphiboles do not have such extreme NbTa enrichments, but NbTa partition coefficients between amphibole and clinopyroxene are remarkably high, ranging from 10 to 85. In apparent contrast with the results on natural mantle amphiboles, recently reported Nb and Ta partition coefficients between amphibole and melts are very low [1,2]. The reason for the apparent contradiction may lie in either the composition of the amphibole or the fluid phase (silica-rich aqueous fluid rather than silicate or carbonate melt). In either case, our observations show that amphibole and mica can be important hosts for Nb and Ta and cannot be ignored in identifying the underlying cause of the nearly universal relative NbTa depletion of subduction-related volcanic rocks. We propose a metasomatic model for creating source regions that are depleted in Nb and Ta relative to Th, U and the LREE. Fluids generated by dehydration of the subducted slab ascend through the overlying mantle wedge and precipitate amphiboles. Highly incompatible elements including Nb and Ta are transferred with the fluid into the wedge where the ‘open-system’ precipitation of amphibole fractionates the trace elements and thus generates low (Nb,Ta)/(Th,U,LREE) ratios in the residual fluid. As this fluid travels further it either directly induces partial melting in hot regions of the wedge or is consumed through ‘closed-system’ crystallization of disseminated amphibole in host peridotite that can later undergo partial melting. In either case the resulting source regions of subduction-related magmas are enriched in highly incompatible trace elements but not in Nb and Ta. This model may be considered either as a complement or as an alternative to published models explaining the chemistry of arc magmas.