LITHIUM ISOTOPE GEOCHEMISTRY OF SEDIMENTS AND HYDROTHERMAL FLUIDS OF THE GUAYMAS BASIN, GULF OF CALIFORNIA

Lithium isotopic compositions of hydrothermally altered sediments of D eep Sea Drilling Project (DSDP) site 477/477A, as well as high tempera ture vent fluids of the Guaymas Basin, have been determined to gain an understanding of lithium exchange during fluid-sediment interaction a t this sediment-covered spreading center. Unaltered turbidite of the b asin has a delta Li-6 value of -10 parts per thousand, 5 - 7 parts per thousand heavier than fresh oceanic basalts. Contact metamorphism ind uced by a shallow sill intrusion results in a decrease of the lithium content of the adjacent sediments and a lighter isotopic value (-8 par ts per thousand). Below the sill, sediments altered by a deep-seated h ydrothermal system show strong depletions in lithium, while lithium is otopic compositions vary greatly, ranging from -11 to +1 parts per tho usand. The shift to lighter composition is the result of preferential retention of the lighter isotope in recrystallized phases after destru ction of the primary minerals. The complexity of the isotope profile i s attributed to inhomogeneity in mineral composition, the tortuous pat hway of fluids, and the temperature effect on isotopic fractionation. The range of lithium concentration and delta Li-6 values for the vent fluids sampled in 1982 and 1985 overlaps with that of the sediment-fre e mid-ocean ridge systems. The lack of a distinct expression of sedime nt input is explained in terms of a flow-through system with continuou s water recharge. The observations on the natural system agree well wi th the results of laboratory hydrothermal experiments. The experimenta l study demonstrates the importance of temperature, pressure, water/ro ck ratio, substrate composition, and reaction time on the lithium isot opic composition of the reacted fluid. High temperature authigenic pha ses do not seem to constitute an important sink for lithium and sedime nts of a hydrothermal system such as Guaymas are a source of lithium t o the ocean. The ready mobility of lithium in the sediment under eleva ted temperature and pressure conditions also has important implication s for lithium cycling in subduction zones.