(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.

本研究测定了深海钻探计划（Deep Sea Drilling Project, DSDP）站位477/477A的热液蚀变沉积物，以及瓜亚斯盆地（Guaymas Basin）的高温喷口流体的锂同位素组成，旨在厘清该覆沉积物扩张中心处流体-沉积物相互作用过程中的锂交换行为。该盆地未蚀变浊积岩的δ⁶Li值为-10‰，比新鲜洋壳玄武岩偏高5~7‰。浅部岩床侵入引发的接触变质作用，导致邻近沉积物的锂含量降低，同时同位素组成更轻（δ⁶Li为-8‰）。岩床下方受深部热液系统蚀变的沉积物，锂元素强烈亏损，而锂同位素组成波动范围极大，介于-11‰至+1‰之间。同位素组成向轻同位素方向偏移，源于原生矿物被破坏后，轻同位素优先赋存于重结晶形成的物相中。同位素剖面的复杂性，可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及δ⁶Li值范围，与无沉积物覆盖的洋中脊系统的对应参数范围重合。沉积物输入未呈现显著的地球化学印记，这一现象可通过具有持续水体补给的穿流系统加以解释。
对自然系统的观测结果与室内热液实验的结论高度吻合。实验研究证实，温度、压力、水岩比、底物组成及反应时间，均对反应后流体的锂同位素组成具有显著影响。高温自生矿物相似乎并非锂的重要储库，而瓜亚斯盆地这类热液系统的沉积物，是海洋中锂的重要来源。锂在高温高压条件下的沉积物中具有良好的迁移性，这一特性对于俯冲带的锂循环研究同样具有重要意义。