Mg-O isotopes trace the origin of Mg-rich fluids in the deeply subducted continental crust of Western Alps
Mg-O isotopes trace the origin of Mg-rich fluids in the deeply subducted continental crust of Western Alps
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Mg-O同位素追踪了西阿尔卑斯山深俯冲大陆地壳中富镁流体的起源
DOI：
10.1016/j.epsl.2016.09.010
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发表时间：
2016-12-15
影响因子：
5.3
通讯作者：
Gong, Ying-Zeng
中科院分区：
文献类型：
--
作者：
Chen, Yi-Xiang;Schertl, Hans-Peter;Gong, Ying-Zeng
流体对俯冲带板幔界面的物质传递起着重要作用。然而，通常很难追踪俯冲板片中特定来源的流体，特别是那些来自蛇纹岩脱水的流体。西阿尔卑斯山Dora-Maira的含柯石英白片岩的特征是相对于围岩具有强烈的镁富集，这需要富镁流体渗入表壳岩。为了确定富镁流体的来源，我们对白片岩及相关岩石进行了全岩Mg、O同位素、锆石U-Pb年龄和O同位素的综合研究。白片岩中的锆石显示两组U-Pb年龄，分别为262 Ma和34 Ma，分别为残留和新生域。残留岩浆域的二叠纪U-Pb年龄与寄主变花岗岩的原岩年龄一致，表明它们共同的原岩为二叠纪花岗岩。第三纪的U-Pb年龄发生在柯石英变质域，与已知的超高压变质作用的年龄。变质域的δ O-18值为5.8-6.8 ‰，而残余岩浆域的δ 180值较高，接近10 ‰。如此高的δ O-18值也是变质花岗岩的特征，表明白片岩原岩经历了变质流体的交代作用，其δ O-18值较低，约为千分之2-4。白片岩大多具有-0.07至0.72千分之一的全岩三角洲Mg-26值，显著高于-0.54至0.11千分之一的围岩三角洲Mg-26值。因此，变质流体不仅富Mg，而且Mg同位素也较重。它们可能是俯冲通道中蛇纹岩中的滑石、叶蛇纹石等富镁含水矿物在板幔界面发生断裂的产物。因此，地幔楔状蛇纹岩在陆壳俯冲折返过程中的脱水作用可以为弧下深部地壳岩石的交代提供富镁流体。(C)© 2016 Elsevier B. V.版权所有。
Fluids are important for mass transfer at the slab-mantle interface in subduction zones. However, it is usually difficult to trace fluids from specific sources in a subducting slab, especially those derived from dehydration of serpentinite. Coesite-bearing whiteschist at Dora-Maira in the Western Alps is characterized by strong Mg enrichment relative to the country rocks, which requires infiltration of Mg rich fluids into the supracrustal rock. In order to constrain the origin of such Mg-rich fluids, we have performed an integrated study of whole-rock Mg and O isotopes, zircon U-Pb ages and O isotopes for the whiteschist and related rocks. Zircons in the whiteschist show two groups of U-Pb ages at similar to 262 Ma and 34 Ma, respectively, for relict and newly grown domains. The Permian U-Pb ages of relict magmatic domains are consistent with the protolith age of host metagranite, suggesting that their common protolith is the Permian granite. The Tertiary U-Pb ages occur in coesite-bearing metamorphic domains, consistent with the known age for ultrahigh-pressure metamorphism. The metamorphic domains have delta O-18 values of 5.8-6.8 parts per thousand, whereas the relict magmatic domains have high 8180 values of similar to 10 parts per thousand. Such high delta O-18 values are also characteristic of the metagranite, indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low delta O-18 value of similar to 2-4 parts per thousand. The whiteschist mostly has whole rock delta Mg-26 values of -0.07 to 0.72 parts per thousand, considerably higher than country-rock delta Mg-26 values of -0.54 to -0.11 parts per thousand. Thus, the metamorphic fluids are not only rich in Mg but also heavy in Mg isotopes. They were probably derived from the breakdown of Mg-rich hydrous minerals such as talc and antigorite in serpentinite at the slab-mantle interface in the subduction channel. Therefore, the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the Mg-rich fluids responsible for the metasomatism of crustal rocks at subarc depths. (C) 2016 Elsevier B.V. All rights reserved.