Lithium isotopic composition and concentration of the upper continental crust

摘要

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, delta(7)Li, and chemical weathering (as measured by the Chemical Index of Alteration (CIA)), and delta(7)Li and the clay content of shales (as measured by Al2O3/SiO2), reflect uptake of heavy Li from the hydrosphere by clays. S-type granites from the Lachlan fold belt (-1.1 to -1.4parts per thousand) have delta(7)Li indistinguishable from their associated sedimentary rocks (-0.7 to 1.2parts per thousand), and show no variation in delta(7)Li throughout the differentiation sequence, suggesting that isotopic fractionation during crustal anatexis and subsequent differentiation is less than analytical uncertainty (+/-1parts per thousand, 2sigma). The isotopically light compositions for both I- and S-type granites from the Lachlan fold belt (-2.5 to + 2.7 parts per thousand) and loess from around the world (-3.1 to + 4.5parts per thousand) reflect the influence of weathering in their source regions. Collectively; these lithologies possess a limited range of Li isotopic compositions (delta(7)Li of -5parts per thousand to + 5parts per thousand), with an average (delta(7)Li of 0 +/- 2parts per thousand at 1sigma) 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 (delta(7)Li of + 4 +/- 2parts per thousand), 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 (2sigma), based on the average loess composition and correlations between insoluble elements (Ti, Nb, Ta, Ga and Al2O3, Th and HREE) and Li in shales. This value is somewhat higher than previous estimates (similar to20 ppm), but is probably indistinguishable when uncertainties in the latter are accounted for. Copyright (C) 2004 Elsevier Ltd.