Lithium Isotope Studies of Oceanic Crust and Convergent Margin Processes

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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.