Lithium Isotope Studies of Oceanic Crust and Convergent Margin Processes

小行星9314708 DSDP站点477/477 A的热液蚀变沉积物的锂同位素组成，以及瓜伊马斯盆地的高温喷口流体，已确定获得在这个沉积物覆盖的扩张中心的流体-沉积物相互作用过程中的锂交换的理解。 该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。 浅岩床侵入体诱发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）。 在岩床之下，被深部热液系统蚀变的沉积物显示出Li的强烈亏损，而同位素组成变化很大，范围从-11到+10/100。 向较轻成分的转变是原生矿物破坏后较轻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是由于矿物成分的不均匀性、流体的曲折通道和温度对同位素分馏的影响。 1982年和1985年取样的喷口流体的Li浓度和6Li值的范围与无沉积物的洋中脊系统重叠。 缺乏一个明确的表达泥沙输入的解释方面的一个连续的水补给的流通系统。 对自然系统的观察与实验室热液实验的结果吻合得很好。 在高温高压条件下，沉积物中Li的易迁移性对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.