Lithium Isotope Studies of Oceanic Crust and Convergent Margin Processes

DSDP 477/477A点热液蚀变沉积物的锂同位素组成，以及Guaymas盆地的高温喷口流体，确定了在该沉积物覆盖扩张中心流体-沉积物相互作用过程中的锂交换。盆地未蚀变浊积岩的6Li值为-10 0/100，比新鲜海洋玄武岩重5 ~ 7 0/100。浅层侵入引起的接触变质作用导致沉积物中Li含量降低，同位素值较轻（- 8 /100）。在基底下，深部热液系统蚀变的沉积物显示出强烈的Li消耗，而同位素组成变化很大，范围在-11到+1 0/100之间。向较轻组成的转变是原生矿物破坏后较轻同位素在重结晶相中优先保留的结果。矿物组成的不均匀性、流体路径的曲折以及温度对同位素分馏的影响是导致同位素剖面复杂性的主要原因。1982年和1985年取样的喷口流体的Li浓度和6Li值范围与无沉积的洋中脊系统重叠。泥沙输入缺乏明确的表达可以用连续补水的水流系统来解释。对自然系统的观测与室内热液实验的结果吻合得很好。高温高压条件下沉积物中Li的动态迁移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.