Barium isotope variation during granitic magma differentiation: implications for the discrimination of crystal fractionation and fluid-magma interaction

Abstract

The genesis and compositional diversity of granites are primarily controlled by multiple processes, including partial melting and magma differentiation. However, discriminating the individual effects of processes such as partial melting, fractional crystallization, and fluid-magma interaction on the evolution of granitic magma remains challenging. Barium isotopes serve as a potentially powerful tracer for discrimination between fractional crystallization and fluid-magma interaction processes, yet the effect of fractionation crystallization on Ba isotope composition variation remains poorly constrained. To tackle this challenge, we measured the Ba isotope compositions of syn-exhumation granites from the Sulu orogen in eastern China. These high-Si granites exhibit significant variations in δ138/134Ba values (–0.69 ‰ to 0.36 ‰). Using phase equilibrium modeling, we estimate that the Ba isotope variation during partial melting process can result in an increase of less than ca. 0.20 ‰ between melts and protoliths, which cannot account for the observed light Ba isotope compositions. Additionally, the content of SiO2 is significantly correlated with contents of other major and trace elements, such as Al2O3, TiO2, Fe2O3t, Ba, Sr and Eu/Eu*, suggesting extensive fractional crystallization in these granites. Moreover, δ138/134Ba values exhibit good correlations with the compositions of such major and trace elements, indicating large Ba isotopic variation during the fractional crystallization of granitic magmas. In contrast, the Zr/Hf, K/Rb, Nb/Ta ratios and REE patterns of the syn-exhumation granites indicate the absence of fluid-magma interaction. These observations indicate that fractional crystallization, rather than fluid-magma interaction, is the primary factor driving significant Ba isotope fractionation in the granites, resulting in δ138/134Ba values as low as −0.69 ‰. Using fractional crystallization modelling with rhyolite-MELTS, we demonstrate that the fractional crystallization of K-feldspar, biotite and muscovite controls the Ba isotope variation of granites, consistent with the petrological observations. Combined with previous results from leucogranites and strongly fractionated granites, we document that it should be taken caution when using Ba isotope compositions to trace the fluid-magma interaction. While crystal fractionation can lead to δ138/134Ba values as low as ca. −0.7 ‰ in granites, values lower than this can be more unambiguously attributed to fluid-magma interaction during the late-stage differentiation of granitic magma.