Trace element fractionation during fluid-induced eclogitization in a subducting slab: trace element and Lu-Hf-Sm-Nd isotope systematics
Trace element fractionation during fluid-induced eclogitization in a subducting slab: trace element and Lu-Hf-Sm-Nd isotope systematics
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DOI：
10.1016/j.epsl.2004.09.009
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发表时间：
2004-11-15
影响因子：
5.3
通讯作者：
Schenk, V
中科院分区：
文献类型：
--
作者：
John, T;Scherer, EE;Schenk, V
赞比亚中部的辉长岩和榴辉岩在空间上密切相关，代表了缝合带中俯冲洋壳的遗迹。榴辉岩形成温度为630-690℃，压力为2.6-2.8 Gpa，Lu-Hf年龄为607+/-14~659+/-14 Ma，表明俯冲活动至少持续24 Ma。辉长岩和榴辉岩的微量元素和同位素组成从来自下洋壳的不相容亏损辉长岩到富集洋岛玄武岩的辉长岩。几个榴辉岩显示轻稀土元素从重稀土和高场强元素中大量分馏，这种影响不是岩浆成因的，但肯定是变质过程中流体通过岩石造成的。在一些样品中，流体通道被榴辉岩相矿物的矿脉所标志。石榴石-基于Sm-ND(相对活动性)和Lu-Hf(相对不活动性)体系的全岩年龄是相同的，与榴辉岩化过程中轻稀土元素的分馏一致。使用流体-矿物分配系数的模型表明，分馏的岩石与相当于其质量80%的流体发生了反应。如此大量的流体最有可能的来源是俯冲板块的蛇纹岩岩石圈地幔。赞比亚榴辉岩及其脉体代表了通过俯冲洋壳的残余流体通道，并为板块内的沟道化流体流动和元素迁移提供了直接证据。干燥的、亚稳定的板片辉长岩在流体渗入时转变为榴辉岩，并伴随着流体活动元素的迁移，可能是在弧状岩浆中产生板片成分的原因。(C)2004爱思唯尔B.V.保留所有权利。
Spatially closely associated gabbros and eclogites of central Zambia represent relics of subducted oceanic crust in a suture zone. The eclogites, which formed at 630-690 degreesC and 2.6-2.8 GPa, yield Lu-Hf ages between 607 +/- 14 and 659 +/- 14 Ma, suggesting that subduction was active for at least 24 Myr. The trace element- and isotope compositions of the gabbros and eclogites range from those of incompatible-element depleted gabbros from the lower oceanic crust to those of enriched ocean-island basalts. Several eclogites display a large fractionation of the light rare earth elements from heavy rare earth- and high field strength elements, an effect that cannot be of magmatic origin but must have resulted from the passage of fluids through the rocks during metamorphism. In some samples, fluid pathways are marked by veins of eclogite facies minerals. Garnet-whole rock ages based on the Sm-Nd (relatively mobile) and Lu-Hf (relatively immobile) systems are identical, consistent with light rare earth elements being fractionated during eclogitization. Modeling using fluid-mineral partition coefficients suggests that the fractionated rocks have reacted with an amount of fluid equal up to 80% of their mass. The most likely source for such a large volume of fluid is the serpentinized lithospheric mantle of the subducting slab. The Zambian eclogites and their veins represent relict fluid pathways through subducted oceanic crust and provide direct evidence for channelized fluid flow and element transport within a slab. The transformation of dry, metastable slab gabbros to eclogites upon fluid-infiltration, accompanied by the transport of fluid-mobile elements, could be responsible for generating the slab component in arc magmas. (C) 2004 Elsevier B.V. All rights reserved.