IODP U1502B钻孔高温蚀变玄武岩的锂同位素特征及对南海初始扩张期热液流体循环的指示

Li isotope characteristics of high-temperature altered basalts from IODP Hole U1502B and its implication for seafloor hydrothermal circulation at the early spreading stage in South China Sea

锂同位素体系是示踪水-岩相互作用的有力工具,但关于其在洋壳高温蚀变过程中同位素分馏行为的认识仍不清晰。本文对国际大洋发现计划(IODP)U1502B钻孔的9个高温蚀变玄武岩开展了岩相学、全岩主、微量元素和锂同位素地球化学研究。结果表明,与新鲜洋中脊玄武岩相比,9个样品的Li含量偏高(4.8×10^(-6)~11.6×10^(-6)),δ7Li值偏轻(-3.8‰~+1.4‰)。岩相学与全岩地球化学特征表明,洋壳高温蚀变过程中产生的绿泥石矿物导致锂同位素分馏、δ7Li值变轻,后期的低温海水蚀变作用使得钻孔上部出现Li的局部富集。蚀变玄武岩的锂同位素特征指示,该钻孔的热液流体循环经历了一个较完整的演化过程,蚀变流体以高温热液为主且含有少量海水;随着取样深度的增加,玄武岩蚀变程度逐渐降低。

The Li isotopic system is a powerful tool to trace water-rock interaction,however,the understanding of Li isotopic fractionation behavior in the process of oceanic crust alteration at high temperature is still unclear.In this study,we have carried out petrography,major and trace elemental geochemistry,and Li isotopic geochemical studies of 9 high-temperature altered basalt samples from the International Ocean Discovery Program(IODP)Hole U1502B.These altered basalts have relatively higher Li contents(4.8×10^(-6)-11.6×10^(-6))but lighter isotopic compositions(-3.8%o~+1.4%o)comparing to those of fresh MORBs.The petrographyic and bulk rock geochemical characteristics of samples show that the chlorites formed in the process of oceanic crust alteration at high temperature resulted in the Li isotopic fractionation and thus those samples have relatively light'Li values,and Li was enriched locally in altered basalts from the upper part of Hole U1502B by the later-stage seawater alteration.Furthermore,the Li isotopic characteristics of altered basalts of Hole U1502B suggest that the hydrothermal circulation system had experienced a relatively complete evolution process and it was dominated by the high-temperature hydrothermal fluid with a small amount of seawater.The alteration degrees of basalts decrease with the increasing sampling depth in Hole U1502B.