(Table 1) Lithium contents and isotopic composition in sediments from DSDP Holes 64-477 and 64-477A

Lithium isotopic compositions of hydrothermally altered sediments of Deep Sea Drilling Project (DSDP) site 477/477A, as well as high temperature vent fluids of the Guaymas Basin, have been determined to gain an understanding of lithium exchange during fluid-sediment interaction at this sediment-covered spreading center. Unaltered turbidite of the basin has a d6Li value of -10%, 5-7% heavier than fresh oceanic basalts. Contact metamorphism induced by a shallow sill intrusion results in a decrease of the lithium content of the adjacent sediments and a lighter isotopic value (-8%). Below the sill, sediments altered by a deep-seated hydrothermal system show strong depletions in lithium, while lithium isotopic compositions vary greatly, ranging from -11 to +1%. 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 lithium concentration and d6Li 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. The observations on the natural system agree well with the results of laboratory hydrothermal experiments. The experimental study demonstrates the importance of temperature, pressure, water/rock ratio, substrate composition and reaction time on the lithium isotopic composition of the reacted fluid. High temperature authigenic phases do not seem to constitute an important sink for lithium and sediments of a hydrothermal system such as Guaymas are a source of lithium to the ocean. The ready mobility of lithium in the sediment under elevated temperature and pressure conditions also has important implications for lithium cycling in subduction zones.本研究对深海钻探计划（Deep Sea Drilling Project, DSDP）477/477A站位经热液蚀变的沉积物，以及瓜伊马斯盆地（Guaymas Basin）高温喷口流体的锂同位素组成进行了测定，以期阐明该覆盖型扩张中心处流体-沉积物相互作用过程中的锂交换机制。该盆地未蚀变的浊积岩δ⁶Li值为-10‰，较新鲜洋壳玄武岩偏重5~7‰。浅成岩床侵入引发的接触变质作用会导致邻近沉积物的锂含量降低，且同位素值偏轻（-8‰）。岩床下方，受深部热液系统蚀变的沉积物锂含量显著亏损，而锂同位素组成变化范围极大，介于-11‰至+1‰之间。同位素组成向轻同位素偏移的原因是，原生矿物被破坏后，轻同位素优先被保留在重结晶相之中。同位素剖面的复杂性归因于矿物组成不均一性、流体运移的曲折路径，以及温度对同位素分馏的影响。 1982年与1985年采集的喷口流体的锂浓度范围与δ⁶Li值范围，与无沉积物覆盖的洋中脊系统的对应参数区间重叠。本次未观测到沉积物输入的显著特征，这一现象可通过具有持续水补给的穿流系统（flow-through system）得到解释。 对自然系统的观测结果与实验室热液实验的结果吻合良好。本实验研究证实，温度、压力、水岩比、基底组成以及反应时间，均会对反应后流体的锂同位素组成产生重要影响。高温自生矿物相似乎并非锂的重要储库，而瓜伊马斯盆地这类热液系统的沉积物，实则是海洋锂的重要来源。锂在高温高压条件下于沉积物中具备良好的迁移能力，这一特性对俯冲带（subduction zones）的锂循环过程亦具有重要启示意义。