A preliminary study on boron isotope fractionation of major rock-forming minerals in granite

We present boron concentrations and boron isotopic compositions of whole-rocks and major rock-forming minerals (biotite, plagioclase, quartz) from two granites in South China. Biotite has highest boron concentrations, and plagioclase has medium boron concentrations. Quartz has very low boron concentrations. Boron in granite is predominantly present in the biotite and plagioclase structure replaced Si and/or Al. Trace boron in quartz may concentrate in the fluid inclusions. Biotite has lowest boron isotopic compositions and quartz has highest boron isotopic compositions. This study firstly proves that large differences of boron isotopic compositions exist between rock-forming minerals in granites. Boron isotopic fractionation between biotite and plagioclase is -9.3‰ for the Dadongshan granite and -6.6‰ for the Qianlishan granite, respectively. Boron isotopic fractionation between biotite and quartz is -9.9‰ for the Dadongshan granite and -10.2‰ for the Qianlishan granite, respectively. Combining oxygen isotopic data, we suggest the large boron isotopic differences between minerals may be induced by post-magma hydrothermal alteration. The T-dependent fractionation factor of boron isotopes between silicates containing tetrahedrally coordinated boron and neutral-acid fluids is approximated by the equation: 10001nα_{silicate-fluid} = -11.19 × (10³/T[K] ) + 5.09. The main fractionation effect is due to the change from trigonal coordination of boron in fluids to tetrahedrally coordinated boron in silicates.