Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China
Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China
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变质流体活动过程中的硅同位素分馏：中国大别造山带榴辉岩和超高压脉的制约
DOI：
10.1016/j.chemgeo.2020.119550
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发表时间：
2020-05
期刊：
影响因子：
3.9
通讯作者：
Huang Fang
中科院分区：
文献类型：
--
作者：
Li Yuanhong;Yu Huimin;Gu Xiaofeng;Guo Shun;Huang Fang
为了解俯冲带变质流体活动过程中硅同位素的分馏，对大别造山带港河和花凉亭地区两个高压-超高压榴辉岩脉系的硅同位素组成进行了研究。结果表明，硅同位素在两个地区的脉状榴辉岩和寄主榴辉岩之间均发生了明显的分馏。岗河榴辉岩的δ 30 Si值为-0.50 ‰ ~-0.39 ‰，高于绿辉石绿帘石脉的δ 30 Si值（-0.63 ± 0.07‰）。花凉亭榴辉岩的δ 30 Si值介于-0.36 ‰ ~-0.29 ‰之间，花凉亭多期脉的δ 30 Si值变化较大，介于-0.45 ‰ ~+0.05‰之间，反映了变质流体演化和脉形成过程中Si同位素的明显分馏。矿物中重Si同位素的富集顺序为δ 30硅白云母（−0.01至+0.13‰）δ 30硅石英（−0.14至+0.10‰）> δ 30硅绿辉石（−0.63至−0.33‰）<$δ 30硅绿帘石（−0.60至−0.30‰）<$δ 30硅蓝晶石（−0.42至−0.28‰）> δ 30锡榴石（−0.92至−0.44‰）。变质矿物间硅同位素分馏与第一性原理计算的平衡硅同位素分馏因子基本一致。这些结果表明，脉矿物之间可能处于Si同位素平衡状态，华凉亭脉的δ 30 Si与SiO2含量呈线性相关，但斜率大于岩浆分异趋势。还考虑到矿脉的矿物学，我们得出结论，这种线性关系反映了矿脉的Si同位素特征上的矿物成分的顺序变化。随着流体结晶作用的继续，脉状体SiO2含量和δ 30 Si值增加，流体δ 30 Si值也随之增加。结果表明，硅同位素数据可以用来制约俯冲带的元素再循环和变质流体活动。
To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone, this study presents the Si isotopic compositions of two high- to ultrahigh-pressure (HP–UHP) eclogite–vein systems from the Ganghe and Hualiangting areas in the Dabie orogen, eastern China. The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas. The δ30Si values of Ganghe eclogites range from −0.50‰ to −0.39‰, higher than that of the studied omphacite−epidote vein (−0.63 ± 0.07‰). The Hualiangting eclogites have δ30Si values of −0.36‰ to −0.29‰, whereas the δ30Si values of the Hualiangting multi-stage veins show greater variation, from −0.45‰ to +0.05‰, revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples, the enrichment of heavy Si isotopes in minerals follows the order of δ30Siphengite(−0.01 to +0.13‰) ≈ δ30Siquartz(−0.14 to +0.10‰) > δ30Siomphacite (−0.63 to −0.33‰) ≈ δ30Siepidote(−0.60 to −0.30‰) ≈ δ30Sikyanite(−0.42 to −0.28‰) > δ30Sigarnet(−0.92 to −0.44‰). The Si isotope fractionation between metamorphic minerals was generally consistent with the equilibrium Si isotope fractionation factors calculated by the first-principles methods. These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The δ30Si of the Hualiangting veins are linearly correlated to SiO2contents with a steeper slope than that of the magma differentiation trend. By also considering the mineralogy of the veins, we conclude that this linear relationship reflects the sequential variation of mineral composition on the Si isotope signature of the veins. The SiO2content and δ30Si of veins increased with continuing crystallization from the fluid, and the δ30Si of the fluid also increased. The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluids activities in the subduction zones.
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