A LITHIUM ISOTOPE STUDY OF HOT-SPRINGS AND METABASALTS FROM MIDOCEAN RIDGE HYDROTHERMAL SYSTEMS

The Li isotopic compositions of seawater and of fresh and altered basalts are distinct and therefore applicable to the study of the hydrothermal processes in the oceanic crust and the Li balance in the ocean. High-temperature fluids from seven vents on the East Pacific Rise (EPR) at 21-degrees-N and 11-degrees-13-degrees-N have deltaLi-6 values ranging between -6 and -11 parts per thousand, i.e., 3-7 parts per thousand heavier than fresh basalt values. The intervent variations in Li concentration and isotopic composition correlate well with water/rock ratios. No temporal change in the isotopic value was observed between 1981 and 1985. Metabasalts show both Li depletion and enrichment relative to fresh basalt. They display light Li isotopic values which reflect incorporation of Li mobilized from fresh basalt. From these observations we conclude that the Li isotopic composition of submarine hot springs is controlled by a path-dependent process involving dissolution of primary minerals and precipitation of Li with alteration phases. DeltaLi-6 values of fluids from the Mid-Atlantic Ridge (-6 to -8 parts per thousand) fall in the range of the EPR, indicating similar reaction controls at the two ridge systems. The lack of Li-7 enrichment in the fluids from slower spreading ridges indicates that Li is not recycled from older weathered crust. Thus the difference between the He-3 and Sr-87/Sr-86 based hydrothermal flux and the crustal transfer rate of Li cannot be reconciled by the inclusion of secondary Li from older crust.