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摘要:
洋壳自洋中脊形成到俯冲进入地幔之前,与流体(如海水和热液流体)在海底表面及洋壳内部可以发生广泛的水-岩相互作用,通过对洋壳蚀变过程中元素迁移和同位素分馏行为的研究,可以帮助我们认识海底热液循环系统,探究地球表层和深部的物质及能量流通。锂(Li)元素对流体活动敏感,在很多地质过程中(如风化作用、海水及热液蚀变等)同位素分馏显著,因此其含量和同位素比值变化可以记录洋壳蚀变过程中的重要信息。但由于蚀变洋壳的直接测试数据仍很匮乏,已有的Li元素和同位素数据解释存在较大争议,导致关于洋壳蚀变过程中Li元素迁移和同位素分馏的机制尚未达成共识。本文主要汇总了近年来针对大洋钻探获取的基岩岩芯Li同位素行为研究资料,探讨了在玄武岩蚀变和深海橄榄岩蛇纹石化过程中影响Li元素迁移和同位素分馏的主要因素(如蚀变温度、蚀变流体的化学组成、水-岩比值、次生矿物沉淀等),并进一步提出近期工作可以在以下方面加强:①继续完善Li储库和提高分析测试精度;②进行不同空间尺度下的Li同位素研究;③关注动力学分馏对高温蚀变过程中Li同位素行为的影响;④开展Li同位素与其他同位素体系的联用。
Abstract:Before subduction into the mantle, the oceanic crust formed at mid-ocean ridges would undergo fluid-rock interaction on the seafloor and within the crust. Study in this regard can enhance our understanding of the seafloor hydrothermal system and crust-mantle recycling. Lithium (Li) is strongly mobilized by hydrous fluids and has significant isotopic fractionation during many geological processes (e.g. weathering, seawater, and hydrothermal alteration), thus the variation in Li content and isotopic ratio provide information of the oceanic crust alteration. At present, geochemical data of altered oceanic crust are still lack, and the interpretations of available Li data are often controversial, which caused no consensus on the mechanism of the alteration process. We summarized the Li data of altered basalts and serpentinized peridotites from oceanic drilling cores, discussed the main factors controlling Li behaviors during the alteration process (e.g., the temperature of fluid-rock reaction, chemical composition of fluid, water-rock ratio, secondary mineral precipitation), and suggested that future studies shall be strengthened in the following directions: (1) keep adding new Li isotope data into the geochemical reservoirs and improving the accuracy of analysis; (2) conduct studies on Li isotopes at different spatial scales; (3) evaluate the effects of both equilibrium and kinetic fractionation when considering the high-temperature alteration processes; (4) combine Li and other isotope systems that display similar behaviors during the oceanic crust alteration.
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Key words:
- lithium /
- element behavior /
- isotope fractionation /
- the alteration of oceanic crust
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图 1 不同环境下的Li同位素组成示意图[14]
Figure 1.
表 1 大洋钻探钻孔岩芯中低温蚀变玄武岩的Li含量和δ7Li值汇总
Table 1. Compilation of Li content and δ7Li values of low-temperature-altered basalts from ocean drilling cores
钻孔编号 Li含量
/10-6平均Li含量
/10-6δ7Li值
/‰平均δ7Li值
/‰数据来源 ODP504B 5.62~8.79 5.90 4.80~11.80 6.75 Chan等[1] ODP896A 4.00~7.9 6.31 4.40~12.60 8.17 Chan等[1] IODP1256C 2.87~9.25 6.00 1.06~5.28 3.02 Gao等[18] IODP1256D 4.17~13.1 5.58 2.24~6.13 4.57 Gao等[18] DSDP417A 12.10~71.40 30.84 0.80~9.00 5.37 Seyedali等[17] DSDP417D 7.50~18.60 11.94 −0.80~7.10 3.73 Seyedali等[17] DSDP418A 5.80~18.30 12.53 −1.70~12.90 1.85 Seyedali等[17] 表 2 大洋钻探钻孔岩芯中高温蚀变玄武岩的Li含量和δ7Li值汇总
Table 2. Compilation of Li contents and δ7Li values of high-temperature-altered basalts from ocean drilling cores
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