Lithium Isotope Studies of Oceanic Crust and Convergent Margin Processes

9314708沈阳盆地477/477A点热液蚀变沉积物和瓜伊马斯盆地高温喷口流体的陈锂同位素组成，确定了该沉积扩张中心流体-沉积物相互作用过程中锂的交换。盆地未蚀变浊积岩的6Li值为-100/100，比新鲜大洋玄武岩重5~70/100。基底浅侵入引起的接触变质作用使沉积物中Li含量降低，同位素值变轻(-80/100)。在基床以下，深部热液系统蚀变的沉积物表现出强烈的Li亏损，而同位素组成变化很大，从-11到1 0/100。向较轻成分的转变是在原生矿物被破坏后，较轻的同位素优先保留在重结晶相中的结果。同位素剖面的复杂性归因于矿物组成的不均质性、流体的曲折路径以及温度对同位素分馏的影响。1982年和1985年采集的喷口流体的Li浓度和6Li值范围与无沉积物的大洋中脊系统的范围重叠。泥沙输入缺乏明显的表达方式，可以用连续补水的流过系统来解释。对自然系统的观察与实验室水热实验的结果很好地吻合。高温高压条件下Li在沉积物中的易迁移性对其在扩张中心的热液通量和俯冲带的Li循环具有重要意义。

9314708 CHAN Lithium isotopic compositions of hydrothermally altered sediments of DSDP site 477/477A, as well as high temperature vent fluids of the Guaymas Basin, have determined to gain an understanding of Li exchange during fluid-sediment interaction at this sediment-covered spreading center. Unaltered turbidite of the basin has a 6Li value of -10 0/100, 5 to 7 0/100 heavier than fresh oceanic basalts. Contact metamorphism induced by a shallow sill intrusion results in a decrease of Li content of the sediment and a lighter isotopic value (- 8 0/100). Below the sill, sediments altered by a deep- seated hydrothermal system show strong depletions in Li, while isotopic compositions vary greatly, ranging from -11 to +1 0/100. The shift to lighter composition is the result of preferential retention of the lighter isotope in recrystallized phases after destruction of the primary minerals. The complexity of the isotope profile is attributed to inhomogeneity in mineral composition, the tortuous pathway of fluids and the temperature effect on isotopic fractionation. The range of Li concentration and 6Li values for the vent fluids sampled in 1982 and 1985 overlaps with that of the sediment-free mid-ocean ridge systems. The lack of a distinct expression of sediment input is explained in terms of a flow-through system with continuous water recharge. Observations on the natural system agree well with the results of a laboratory hydrothermal experiment. The ready mobility of Li in the sediment under elevated temperature and pressure conditions has important implications for its hydrothermal flux at spreading centers and Li cycling in subduction zones.