Fe-Mg Isotopic Compositions of Altered Oceanic Crust and Subduction-Zone Fluids
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摘要: 贫碳酸盐的蚀变洋壳具有与新鲜洋中脊玄武岩一致的Mg同位素组成，说明低温和高温洋壳蚀变不会导致Mg同位素分馏.大别山港河和花凉亭的早期变质脉比榴辉岩具有偏高的δ56Fe-δ26Mg值，而且早期到晚期变质脉的δ56Fe-δ26Mg值逐渐降低.这些结果说明，在流体-岩石反应和流体演化过程中，Fe-Mg同位素发生了显著的分馏，且矿物溶解-再沉淀是同位素分馏的控制因素.相比洋中脊玄武岩，蚀变洋壳和变质脉具有相似或偏高的δ56Fe-δ26Mg值，说明蚀变洋壳脱水产生的流体富集重Fe-Mg同位素，不能解释弧岩浆岩的轻Fe/重Mg同位素组成.因此，弧岩浆岩异常的Fe-Mg同位素组成是熔体提取和富集54Fe-26Mg的蛇纹岩流体交代地幔楔两个过程共同作用的结果.Abstract: The origin of the light Fe and heavy Mg isotope enrichments in arc lavas remains unclear because of the lack of constraints on the Fe-Mg isotope compositions of altered oceanic crust (AOC) and metamorphic fluids in subduction zones. Carbonate-barren AOC has Mg isotope compositions similar to those of fresh mid-ocean ridge basalts, suggesting that low-to-high temperature alteration of oceanic crust by seawater and hydrothermal fluids results in limited Mg isotope fractionation. Fe-Mg isotope measurements show that the early omphacite-epidote veins have higher δ56Fe and δ26Mg compared to the host eclogites and that the δ56Fe and δ26Mg gradually decrease from the early omphacite-epidote through epidote-quartz to the late kyanite-epidote-quartz veins. These results indicate significant Fe-Mg isotope fractionation during fluid-rock interaction and fluid evolution due to the dissolution-precipitation processes of minerals in subduction zones. Compared to mid-ocean ridge basalts, the similar or higher δ56Fe and δ26Mg of AOC and metamorphic veins suggest that AOC-derived fluids are probably enriched in heavy Fe-Mg isotopes. Thus, contribution from AOC-derived fluids is unlikely to explain the light Fe and heavy Mg isotope compositions of arc lavas. We propose that the Fe-Mg isotope anomaly of arc lavas may result from a combination of prior melt depletion and addition of serpentinite-derived 54Fe-26Mg-rich fluids into the overlying mantle wedge.
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Key words:
- Fe-Mg isotopes /
- altered oceanic crust /
- eclogite /
- metamorphic fluids /
- fluid evolution /
- arc lavas /
- geochemistry
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图 1 IODP 1256钻孔洋壳的蚀变温度(a)，δ18O(b)和δ26Mg(c)的空间变化
蚀变温度、O和Mg同位素数据引自Alt et al.(2010)、Gao et al.(2012)和Huang et al.(2015).灰色条带表示新鲜洋中脊玄武岩的O和Mg同位素组成(Harmon and Hoefs, 1995；Teng et al., 2010)
Fig. 1. Down-hole variations in alteration temperatures, δ18O, and δ26Mg of oceanic crust from IODP site 1256
图 2 大别山港河和花凉亭榴辉岩和变质脉的Fe3+/ΣFe、δ26Mg和δ56Fe变化
据Huang et al.(2019).灰色条带表示新鲜洋中脊玄武岩的Fe-Mg同位素组成(Weyer and Ionov, 2007；Teng et al., 2010；Nebel et al., 2013)
Fig. 2. Fe3+/ΣFe、δ26Mg, and δ56Fe in ecoligites and veins at Ganghe and Hualiangting in the Dabie orogen
图 3 大别山港河和花凉亭超高压榴辉岩和变质脉中矿物的Fe-Mg同位素组成
据Huang et al.(2019).黑色正方形表示新鲜洋中脊玄武岩的Fe-Mg同位素组成(Weyer and Ionov, 2007；Teng et al., 2010；Nebel et al., 2013)
Fig. 3. δ26Mg and δ56Fe of minerals from ecoligites and veins at Ganghe and Hualiangting in the Dabie orogen
图 4 大别山花凉亭三期变质脉全岩(a, b)和绿帘石(c, d)的Eu/Eu*、δ26Mg和δ56Fe协变图解
Fig. 4. Eu/Eu*, δ26Mg, and δ56Fe in whole-rocks (a, b) and epidotes (c, d) of multi-stage veins at Hualiangting in the Dabie orogen
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