Magnesium isotopic composition of the deep continental crust
Magnesium isotopic composition of the deep continental crust
复制标题
深部大陆地壳的镁同位素组成
DOI：
10.2138/am-2016-5275
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发表时间：
2016-02
影响因子：
3.1
通讯作者：
Zhang Hongfu
中科院分区：
文献类型：
--
作者：
Liu Shengao;Ke Shan;Liu Yongsheng;Zhang Hongfu
摘要为了约束深部地壳过程中Mg同位素的行为和中下陆壳的Mg同位素组成，对30个高变质地岩和18个麻粒岩包体的复合样品进行了研究。合成物来自中国两个最大的太古宙克拉通的8个不同的高变质地质体，包括13个TTG片麻岩、5个角闪岩、4个长英质、4个中质和4个基性麻粒岩。它们的体积组成变化较大，SiO2在45.7% ~ 72.5%之间，代表了中国东部的中地壳。这些样品的δ26Mg值在−0.40 ~ +0.12‰之间变化，反映了其原岩的非均质性，可能涉及上地壳沉积。新生代汉诺坝玄武岩麻粒岩包体组成也具有多样性，MgO含量在2.95 ~ 20.2%之间。这些捕虏体在800-950°C的高温下平衡，对应于下大陆地壳的深度（> - 30km）。δ26Mg的变化幅度较大，为−0.76 ~−0.24‰。轻Mg同位素组成可能是与同位素轻变质流体（可能是碳酸盐流体）相互作用的结果。结合前人资料，估计中、下地壳的平均δ26Mg值分别为- 0.21±0.07‰和- 0.26±0.06‰。大陆地壳的δ26Mg平均值为- 0.24±0.07‰，与地幔的δ26Mg平均值相近。大陆地壳中Mg同位素的大变化反映了大陆风化作用、地壳深部表壳物质的参与和流体交代作用等多重作用的综合作用。
Abstract To constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the middle and lower continental crust, 30 composite samples from high-grade metamorphic terranee and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranee ill the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO2 ranging from 45.7 to 72.5%, representative of the middle crust beneath eastern China. The δ26Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower continental crust (>30 km). They yield a large δ26Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ26Mg values of the middle and lower continental crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk continental crust is estimated to have an average δ26Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the continental crust reflects the combination of several processes, such as continental weathering, involvement of supracrustal materials in the deep crust, and fluid metasomatism.
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