LITHIUM ISOTOPIC COMPOSITION OF SUBMARINE BASALTS - IMPLICATIONS FOR THE LITHIUM CYCLE IN THE OCEANS

We have measured the Li isotope composition of young, pristine basalts from active ocean ridge crests, of progressively older basalts along a dredging transect, and a limited number of hydrothermally altered basalts. The data significantly extend our limited knowledge of the isotopic abundance ratio of Li in geological material. Fresh mid-ocean ridge basalts have delta-Li-6 values of between -3.4 and -4.7 parts per thousand relative to isotope standard L-SVEC. During low-temperature weathering on the seafloor, the isotopic composition of the rock becomes increasingly heavier due to addition of seawater Li (delta-Li-6 = -32.3 parts per thousand). The oldest (46 m.y.) and most altered rock studied has an isotopic composition of -14 parts per thousand. A linear relationship exists between delta-Li-6 and the inverse of Li concentration, suggesting that Li in weathered basalts can be regarded as a two-component mixture of basaltic Li and seawater-derived Li that has been incorporated in alteration minerals, most likely secondary clays such as smectite and phillipsite. The inferred Li isotopic composition of the alteration endmember indicates an apparent isotopic fractionation factor of 1.019 relative to the seawater source. Thus Li uptake by secondary minerals from the low-temperature weathering process and, by analogy, incorporation in similar authigenic minerals in marine sediments provides a mechanism for preferential removal of the lighter Li isotope from ocean water. However, isotopic fractionation due to authigenic clay formation alone cannot account for the isotopic difference between seawater and its principal sources, unless the hydrothermal flux is comparable to the river flux. Alternatively, more important sinks of Li-6 must exist if the steady state isotopic composition of Li in ocean water is to be maintained.