Geochemistry of the Mariana arc (western Pacific): source composition and processes
Geochemistry of the Mariana arc (western Pacific): source composition and processes
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DOI：
10.1016/0009-2541(89)90124-1
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发表时间：
1989-07
期刊：
影响因子：
3.9
通讯作者：
J. Woodhead
中科院分区：
文献类型：
--
作者：
J. Woodhead
西太平洋的北方马里亚纳群岛是洋内岛弧系统的一个典型例子，不受大陆地壳污染的影响。放射性成因（Sr，Nd，Pb，Hf）和稳定（O，H，S）同位素数据的一套熔岩从这些岛屿，以及稀土和其他微量元素的成分数据相同的样品。这种地球化学综合提供了一个机会，以解决一些悬而未决的问题，有关大洋弧岩石成因，特别是地幔源的性质和过程发生在这一地区。许多争论围绕着经常观察到的富集岛弧熔岩相对于洋中脊玄武岩（MORB），特别是这些结果是否来自于板块衍生成分并入MORB型地幔，或者它们是否反映了与洋岛玄武岩（OIB）有密切关系的源的优先熔融。虽然一些地球化学数据是模糊的，大多数似乎赞成板参与假说，从俯冲沉积物和蚀变洋壳的贡献，微量元素富集模式的马里亚纳弧火山岩表明，代理之间的俯冲板和上覆地幔楔的质量转移是一种流体，它已经浸出的元素从板，而不是直接的部分熔融后者。虽然这种液体的确切成分尚不清楚，但数据表明其中很大一部分可能是含水的。元素分配到这种流体中是高度选择性的（即板片与地幔楔之间的物质传递不是等化学过程），这一结论对壳幔演化的研究具有重要意义。实际上，岛弧熔岩和MORB中HFSE浓度之间存在大量重叠。岛弧中岛屿之间的区域地球化学变化很小，被认为最有可能与俯冲组合性质的不均匀性有关。
The northern Mariana islands in the western Pacific form a prime example of an intra-oceanic island arc system, free from the effects of continental crustal contamination. Radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotope data are presented for a suite of lavas from these islands, together with rare-earth and other trace-element compositional data for the same samples. This geochemical synthesis provides an opportunity to address a number of outstanding questions concerning oceanic arc petrogenesis, in particular the nature of the mantle source and processes occurring within this region.Much debate surrounds the origin of the commonly observed enrichments in island arc lavas relative to mid-ocean ridge basalt (MORB), in particular whether these result from incorporation of slab-derived components into a MORB-type mantle or whether they reflect preferential melting of a source with affinities to oceanic island basalt (OIB). Although some geochemical data are ambiguous, the majority appear to favour the slab involvement hypothesis, with contributions from both subducted sediment and altered oceanic crust.The trace-element enrichment patterns of the Mariana arc volcanics suggest that the agent of mass transfer between the subducting slab and overlying mantle wedge is a fluid, which has leached elements from the slab, rather than a direct partial melt of the latter. Although the exact composition of this fluid is unknown, the data indicate that a large proportion of it may be aqueous. Elemental partitioning into this fluid is highly selective (i.e. mass transfer between the slab and mantle wedge is not an iso-chemical process), a conclusion which has considerable implications for studies of crust and mantle evolution.The commonly cited depletions of high-field-strength elements (HFSE) in island arcs relative to MORB are thought to be largely a product of the data normalization process; in reality substantial overlap exists between HFSE concentrations in arc lavas and MORB.Regional geochemical variations between the islands in the arc are small and are thought most likely to be related to heterogeneity in the nature of the subducting assemblage.