Investigation of magnesium isotope fractionation during granite differentiation: Implication for Mg isotopic composition of the continental crust

摘要

High-precision Mg isotopic analysis was performed on a suite of well-characterized I-type granitoids and associated hornblende and biotite minerals from the Dabie Orogen in central China, to address the behavior of Mg isotopes during granite differentiation. Although these granitoids formed through different degrees of partial melting and fractional crystallization, with large variations in elemental and mineral compositions, their δ~(26)Mg values vary from -0.26 to -0.14 and are indistinguishable within our analytical precision (±0.07‰ 2SD). Coexisting hornblendes and biotites in these granitoids display similar Mg isotopic composition, with δ~(26)Mg ranging from -0.31 to -0.14 in hornblendes and -0.23 to -0.12 in biotites. The inter-mineral fractionation factors (δ~(26)Mg_(Hbl-Bt)=δ~(26)Mg_(Hbl)δδ~(26)Mg_(Bt)) vary from -0.10 to -0.02, with an average=-0.06±0.08 (2SD). The limited inter-mineral fractionation agrees with the theoretic prediction that Mg cations in both hornblende and biotite are octahedrally coordinated with oxygen, which restricts the magnitude of equilibrium isotope fractionation. Overall, data from both bulk granitoids and associated mineral separates suggest that Mg isotope fractionation during I-type granite differentiation is limited.Collectively, granitoids studied here have Mg isotopic composition similar to that of terrestrial basalts and peridotites (δ~(26)Mg=-0.21±0.07 vs. -0.25±0.07; 2SD), confirming that magmatic processes do not significantly fractionate Mg isotopes. The continental crust in the Dabie Orogen, as sampled by these I-type granitoids, has a mantle-like Mg isotopic composition. Given that significant Mg isotope fractionation occurs during chemical weathering processes, Mg isotopes may potentially be used for tracing granite genesis, in particular, if sedimentary materials are involved in granite sources.