Lithium isotopic composition and concentration of the upper continental crust

Abstract

The Li isotopic composition of the upper continental crust is estimated from the analyses of well-characterized shales, loess, granites and upper crustal composites (51 samples in total) from North America, China, Europe, Australia and New Zealand. Correlations between Li, δ 7Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and δ 7Li and the clay content of shales (as measured by Al 2O 3/SiO 2), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (–1.1 to –1.4‰) have δ 7Li indistinguishable from their associated sedimentary rocks (–0.7 to 1.2‰), and show no variation in δ 7Li throughout the differentiation sequence, suggesting that isotopic fractionation during crustal anatexis and subsequent differentiation is less than analytical uncertainty (±1‰, 2σ). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (–2.5 to + 2.7 ‰) and loess from around the world (–3.1 to + 4.5‰) reflect the influence of weathering in their source regions. Collectively, these lithologies possess a limited range of Li isotopic compositions (δ 7Li of −5‰ to + 5‰), with an average (δ 7Li of 0 ± 2‰ at 1σ) that is representative of the average upper continental crust. Thus, the Li isotopic composition of the upper continental crust is lighter than the average upper mantle (δ 7Li of + 4 ± 2‰), reflecting the influence of weathering on the upper crustal composition. The concentration of Li in the upper continental crust is estimated to be 35 ± 11 ppm (2σ), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al 2O 3, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (∼20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for.