Lithium Isotope Fractionation during Granitic Magma Differentiation: Insights from Whole Rock and Mineral Analyses

Abstract Despite extensive studies on whole-rock lithium (Li) isotope geochemistry, the fractionation of Li isotopes among minerals in granitic magmatic systems is still poorly understood. To investigate the magnitude and mechanism of inter-mineral and inter-sample Li isotope fractionation during granitic magma differentiation, we present high-precision Li isotope data from a suite of well-characterized granitoids (quartz diorite, granodiorite, and monzogranite) and their minerals from the Tagong pluton in NE Tibetan Plateau. The whole rocks exhibit δ7Li variation from −1.73 ± 0.15 to 1.30 ± 0.08‰, indicating insignificant Li isotope fractionation at the whole-rock scale. The constituent mafic minerals show δ7Li values varying from −1.68 ± 0.05 to 1.76 ± 0.15‰ in biotite, −2.08 ± 0.15 to 2.53 ± 0.13‰ in amphibole, and −1.72 ± 0.21 to 0.74 ± 0.08‰ in clinopyroxene, demonstrating equilibrium isotope fractionation between these coexisting phases during magma differentiation. The comparable δ7Li values between whole rocks and biotite suggest that crystallization of the Li-richest phase (biotite Li = 139–361 μg/g) is the key factor controlling whole-rock Li isotope compositions. In contrast, quartz, K-feldspar, and plagioclase display significant inter-mineral and inter-sample Li isotope variations with δ7Li ranges of 15.09 ± 0.14 to 26.00 ± 0.33‰, 1.69 ± 0.10 to 8.53 ± 0.01‰, and −2.23 ± 0.08 to 6.50 ± 0.19‰, respectively. Rayleigh fractionation alone cannot fully account for the Li isotope variations in these felsic minerals. Plagioclase in the K-feldspar-rich granodiorites and monzogranites is isotopically lighter and more variable than in the K-feldspar-free quartz diorites, indicating competition for Li isotopes between K-feldspar and plagioclase during simultaneous crystallization. The Li isotope variation in quartz that has low Li content (0.67 to 2.82 μg/g) may be related to the diffusion process. Furthermore, mafic microgranular enclaves, typically observed in granitoid rocks, show whole-rock and mineral Li isotope compositions similar to the counterparts of their host rocks, suggesting that they are of schlieren origin. This provides further evidence for minimal Li isotope fractionation at the whole-rock level. In comparison to the low-silica (SiO2 < 65 wt %) and uniform whole-rock Li isotope characteristics of the granitoid samples studied here, global high-silica (SiO2 > 65 wt %) granitic rocks exhibit highly variable δ7Li values from −3.0 to 8.0‰. This variability can be attributed to the fractional crystallization of felsic minerals, which are more abundant in the high-silica rocks.