Paper No. 201-3
Presentation Time: 8:35 AM
EXTREME LITHIUM ISOTOPIC FRACTIONATION BETWEEN QUARTZ AND GARNET IN LEUCOGRANITES: IMPLICATIONS FOR LI SYSTEM DURING SUBDUCTION AND CRUSTAL MAGMA PROCESSES
We report Li concentrations and isotopic compositions for minerals and bulk rocks of subduction-related garnet-bearing leucogranites from the southeastern margin of the North China craton. Lithium isotopic fractionation between coexisting mineral phases was found to be significant. The five investigated leucogranite samples have low Li concentrations (1.8-5.1 ppm) and considerably variable but much higher δ7Li values (+4.0 to +13.1‰) compared to an average of Li concentration (` 36ppm) and isotopic composition (`2.2‰) for granites worldwide. Quartz has relatively low Li concentrations (~ 1 ppm) but is strongly enriched in 7Li, with δ7Li values ranging from +27.0‰ to +38.5‰. By contrast, garnet has unusually high Li concentrations (27.7-50.4 ppm) and depleted but relatively homogeneous isotopic compositions, with δ7Li values ranging from -1.3‰ to + 0.3‰. Plagioclase has 3.1-4.2 ppm Li and δ7Li values of +1.8‰ to +8.9‰, whereas those for K-feldspar are 0.9-2.8 ppm and +3.4 to +11.4‰, respectively. Biotite has very high Li concentrations (233-260 ppm) and δ7Li values of +5.1‰ to 10.8‰, falling within the range of feldspars. These observations indicate that significant Li isotopic fractionation between minerals could occur during magmatic and/or metamorphic processes. The extreme inter-mineral isotopic fractionations, with Δquartz-plagioclase (= δ7Liquartz - δ7Liplagioclase) of 21.2 to 28.3, Δquartz-k-feldspar of 23.6 to 30.0, Δquartz-biotite of 19.4 to 26.4, and Δ7Liquartz-garnet of 28.3 to 33.7, could be explained by equilibrium isotope fractionation related to the variation of coordination number and the bond strength of Li to its neighboring atoms in different minerals. The observed Li isotope fractionation among the coexisting mineral phases suggests that, large inter-mineral Li isotope fractionation could be expected where garnets are present, in both crust and mantle rocks. This finding thus may have important implications for understanding behavior of the lithium isotopes during subduction and crustal recycling processes.