Subduction zone Hf-anomalies: Mantle messenger, melting artefact or crustal process?
Subduction zone Hf-anomalies: Mantle messenger, melting artefact or crustal process?
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DOI：
10.1016/j.epsl.2011.01.036
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发表时间：
2011-04
影响因子：
5.3
通讯作者：
J. Woodhead;J. Hergt;A. Greig;L. Edwards
中科院分区：
文献类型：
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作者：
J. Woodhead;J. Hergt;A. Greig;L. Edwards
Hf元素亏损俯冲带岩浆的起源进行了研究，使用新的主要和微量元素的数据，从马里亚纳弧堆包体，深海沉积物恢复的DSDP和ODP钻井计划。结果表明，大部分的稀土元素（REE）和铪库存的捕虏体中包含在两种矿物-单斜辉石和钛磁铁矿-和一个典型的辉长岩分馏组合的去除减少亏损Hf相对于稀土元素相邻的地幔标准化微量元素图（通常表示Hf/Hf*）在不断发展的岩浆。从不同的俯冲带，其中散装岩石样品也定义了Hf/Hf* 和岩浆的二氧化硅含量之间的正相关性的各种文献数据提供了这种观察的确认。与REE-HFSE分配的实验研究一致，我们观察到单斜辉石影响分馏岩浆的Hf/Hf* 的能力与其铝含量有关。这种脱钩的Hf从稀土元素在区分弧岩浆表明，散装岩石Hf/Hf* 值，孤立使用时，不太可能提供一个强大的测量源REE-Hf特性，即使当套件过滤，以排除所有，但最镁铁质样品。可能将数据归一化为恒定的分馏程度，并且以这种方式区分源Hf/Hf* 的细微变化，但是大多数现有数据集的大小和质量都不足以尝试这种计算。修改的Hf/Hf* 也被视为地幔熔融过程建模时，有强烈的建议，源的变化不仅受到俯冲沉积物，表现出显着的宽范围的Hf/Hf*，但也俯冲带流体。这些意见删除了一些限制施加在最近的模型，试图调和铪同位素数据与Hf-REE丰度数据在一些弧套房。虽然一个案件可能是参与残留的，次要的，阶段在下行板，流体和沉积物的添加，和主要阶段的作用，在部分熔融，特别是单斜辉石，在地幔楔也可以施加强烈的影响，Hf-稀土元素的关系。
The origin of Hf elemental depletions in subduction zone magmas is investigated using new major- and trace-element data for cumulate xenoliths from the Mariana arc, and deep sea sediments recovered by the DSDP and ODP drilling programmes. Results indicate that most of the rare earth element (REE) and Hf inventory in the xenoliths is contained within two minerals—clinopyroxene and titanomagnetite—and that removal of a typical gabbroic fractionating assemblage reduces the depletion in Hf relative to neighbouring REE on a mantle normalised trace element diagram (commonly denoted Hf/Hf*) in the evolving magmas. Confirmation of this observation is provided by a variety of literature data from different subduction zones in which bulk-rock samples also define a positive correlation between Hf/Hf* and the silica content of the magmas. In agreement with experimental studies on REE–HFSE partitioning, we observe that the ability of clinopyroxene to influence the Hf/Hf* of fractionating magmas is associated with its aluminium content. This decoupling of Hf from the REE in differentiating arc magmas suggests that bulk rock Hf/Hf* values, when used in isolation, are unlikely to provide a robust measure of source REE–Hf characteristics, even when suites are filtered to exclude all but the most mafic samples. It may be possible to normalise data to a constant degree of fractionation, and in this way distinguish subtle changes in source Hf/Hf* but most existing datasets are of neither the size nor quality to attempt such calculations. Modification of Hf/Hf* is also seen when modelling mantle melting processes and there are strong suggestions that source variations are influenced by not only subducted sediment, which exhibits a remarkably wide range in Hf/Hf*, but also subduction zone fluids. These observations remove some of the constraints imposed on recent models that attempt to reconcile Hf isotope data with Hf–REE abundance data in some arc suites. Although a case may be made for the involvement of residual, minor, phases in the downgoing slab, fluid and sediment addition, and the role of major phases during partial melting, in particular clinopyroxene, in the mantle wedge can also exert a strong influence on Hf–REE relationships.