Behavior of lithium and its isotopes during weathering of Hawaiian basalt

We examined the pedogenic behavior of lithium (Li) and its isotopes in Hawaii by sampling same-age lava flows under mean annual rainfall ranging from 18 to 300 cm. Lithium concentrations in these soils vary from 1 to 29 ppm. Whereas Na, K, and Ca are completely leached from the soil at the most humid and severely weathered site, Li, Mg, Si, and Al show significant retention due to their association with secondary clay minerals. In these soils, allochthonous Li delivered in marine and mineral aerosol mixes with basalt-derived Li, modifying the isotopic composition of the Li pool. The ability of soil to retain Li is related to its effective cation exchange capacity, which in turn is governed by rainfall and leaching intensity and their resulting effect on mineralogy. Lithium isotope ratios have a large range (delta(7)Li of -0.4 to 14%), well beyond the intershield or temporal variation of the lavas of Hawaiian volcanoes (2.5 to 5.7%). The most unweathered samples have delta(7)Li (similar to5-6%) comparable to that of Mauna Kea and Mauna Loa lavas. The Li concentrations and isotope ratios together suggest that in arid to subhumid sites there is a net addition of isotopically heavier lithium from marine aerosol but that under greater rainfall there is a net loss of Li, and isotopically light Li is preferentially retained. Thus, during incongruent weathering of primary minerals, Li, especially Li-6, is adsorbed and sequestered by the clay mineral fraction of high cation exchange capacity soils in drier regions but lost from highly weathered, low cation exchange capacity acidic soils in the wetter regions.