Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones
Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones
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DOI：
10.1016/s0016-7037(02)01321-2
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发表时间：
2003-06
影响因子：
5
通讯作者：
D. Rubatto;J. Hermann
中科院分区：
文献类型：
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作者：
D. Rubatto;J. Hermann
锆石从榴辉岩和一个封闭的榴辉岩相脉蒙维索蛇绿岩（西阿尔卑斯山）显示对比鲜明的化学和形态特征和文件的蛇绿岩的演化的不同阶段。榴辉岩中的锆石具有典型的岩浆分带性，重稀土元素（HREE）相对于中稀土元素（MREE）富集，并具有明显的负Eu异常，表明其与斜长石共结晶。这些岩浆锆石记录了163 ± 2 Ma洋壳的形成。与此相反，从静脉锆石包含石榴石，绿辉石和金红石，这表明他们结晶榴辉岩相条件下的包裹体。脉状锆石Th/U < 0.09，无Eu异常，相对于MREE，HREE仅弱富集。这些特征与含石榴子石、无斜长石，即，榴辉岩相共生脉状锆石的年龄为45 ± 1 Ma，是蒙维索蛇绿岩始新世俯冲带变质作用的证据。在脉中，锆石，绿辉石和石榴石的明显共存允许在高压下确定这些矿物之间的一组微量元素分配系数。这组分配可以证明岩石中这些相之间的化学平衡，这些相显示出不太清楚的结构平衡证据。此外，锆石年龄现在可以与变质压力-温度条件的传感器联系起来。矿脉中锆石和金红石的存在是榴辉岩相条件下含水流体中短距离高场强元素（HFSE）迁移率的另一个例子。然而，在含水流体中的Zr和Hf的浓度估计至少是一个因素的10小于原始地幔值。质量平衡计算表明，锆石主机> 95%的散装锆，90%的铪，和25%的U在静脉。锆石是俯冲玄武岩和沉积物中的残留相，温度至少高达800至900 °C。因此，残留在俯冲地壳中的锆石，连同金红石，控制HFSE在解放俯冲带流体/熔体，并可能部分负责负Zr和Hf异常俯冲带岩浆。
The zircons from an eclogite and an enclosed eclogite-facies vein from the Monviso ophiolite (Western Alps) display contrasting chemical and morphologic features and document different stages of the evolution of the ophiolite. The zircons from the eclogite show a typical magmatic zoning and are enriched in heavy rare earth elements (HREEs) over middle rare earth elements (MREEs) and have an accentuated negative Eu anomaly, which indicates that the grains co-crystallised with plagioclase. These magmatic zircons document the formation of oceanic crust at 163 ± 2 Ma. In contrast, zircons from the vein contain inclusions of garnet, omphacite, and rutile, which indicate that they crystallised under eclogite-facies conditions. The vein zircons have Th/U ratios < 0.09, lack Eu anomalies, and are only weakly enriched in HREE with respect to MREE. These features are consistent with a garnet-bearing, plagioclase-free, i.e., eclogite-facies paragenesis. Vein zircons yield an age of 45 ± 1 Ma, which is evidence for Eocene subduction-zone metamorphism of the Monviso ophiolite. In the vein, the apparent coexistence of zircon, omphacite, and garnet permits the determination of a set of trace element distribution coefficients among these minerals at high pressure. This set of partitioning can demonstrate chemical equilibrium among these phases in rocks that show less clear evidence of textural equilibrium. In addition, zircon age can now be linked to sensors of metamorphic pressure-temperature conditions. The presence of zircon and rutile in the vein is another example of high field strength element (HFSE) mobility over short distances in aqueous fluids at eclogite-facies conditions. However, the concentrations of Zr and Hf in the aqueous fluid are estimated to be at least a factor of 10 less than primitive mantle values. Mass balance calculations demonstrate that zircon hosts > 95% of the bulk Zr, 90% of Hf, and ∼25% of U in the vein. Zircon is a residual phase in subducted basalts and sediments up to temperatures of at least 800 to 900 °C. Therefore, residual zircon in subducted crust, together with rutile, control the HFSE in liberated subduction zone fluids/melts and might be partly responsible for negative Zr and Hf anomalies in subduction zone magmas.