Lithium isotopic composition and concentration of the deep continental crust

Abstract

Samples from Archean high-grade metamorphic terranes in China and granulite-facies xenoliths from Australia (Chudleigh and McBride suites) and China (Hannuoba suite) have been analyzed to assess the Li concentrations and isotopic compositions of the middle and lower continental crust, respectively. Thirty composite samples from metamorphic terranes, including tonalite–trondjhemite–granodiorite (TTG) gneisses, amphibolites and felsic to mafic granulites, show a large variation in Li concentrations (5–33 ppm) but a relatively narrow range in δ 7Li values, from + 1.7 to + 7.5 with a mean of + 4.0 ± 1.4 (1 σ). These results suggest that the middle continental crust is relatively homogenous in Li isotopic composition and indistinguishable from the upper mantle. This may be a primary feature or may reflect homogenization of Li isotopes during exhumation of the metamorphic terranes. In contrast, Li isotopic compositions of granulite xenoliths from the lower crust vary significantly, with δ 7Li ranging from − 17.9 to + 15.7. δ 7Li of minerals also shows a very large spread from − 17.6 to + 16.7 for plagioclases and − 14.6 to + 12.7 for pyroxenes. Large Li isotopic variations exist between plagioclase and pyroxene, with pyroxenes (13 out of 14) isotopically equal to or lighter than coexisting plagioclases. Lithium concentrations of granulite xenoliths also vary widely (0.5 to 21 ppm) and are, on average, lower than those of terranes (5 ± 4 vs. 13 ± 6 ppm respectively, 1 σ), consistent with a higher proportion of mafic lithologies and a higher metamorphic grade for the xenoliths. Pyroxene separates from granulite xenoliths have equal or significantly greater Li than coexisting plagioclase. These large Li isotopic variations between minerals and in whole-rock granulite xenoliths mostly reflect diffusion-driven kinetic isotopic fractionation during the interactions of xenoliths with host magma. Only those xenoliths that reach inter-mineral isotopic equilibria are likely to preserve the initial Li isotopic signatures of the lower crust. Eight such equilibrated samples have δ 7Li from − 14 to + 14.3, with a concentration weighted average of + 2.5, which is our best estimate of the average δ 7Li of the lower continental crust. The substantial isotopic heterogeneity of the lower crust may reflect the combined effects of isotopic fractionation during granulite-facies metamorphism, diffusion-driven isotopic fractionation during igneous intrusion and variable protolith compositions. Consistent with previous B elemental and O isotopic studies, the Li isotopic heterogeneity in the lower crust indicates that pervasive fluid migration and equilibration have not occurred. Using all data for granulite xenoliths, the Li concentration of the lower crust is estimated to be ∼ 8 ppm. Together with previous estimates of Li concentration in the upper and middle crust, the average Li concentration of the bulk continental crust is estimated to be 18 ppm, which is similar to previous estimates. The average Li isotopic composition of the continental crust is estimated to be + 1.2, which is isotopically lighter than upper mantle and may reflect the loss of isotopically heavy Li from the continents during weathering and metamorphic dehydration.