Experiments reveal enrichment 11B in granitic melt resulting from tourmaline crystallisation

Tourmaline is the most common boron-rich mineral in magmatic systems. In this study, we determined experimentally the fractionation of boron isotopes between granitic melt and tourmaline for the first time. Our crystallisation experiments were performed using a boron-rich granitic glass (B2O3 approximate to 8 wt. %) at 660-800 degrees C, 300 MPa, and a(H2O) = 1, in which tourmaline occurs as the only boron-hosting mineral. Our experimental results at four different temperatures show a small and temperature-dependent boron isotope fractionation between granitic melt and tourmaline (Delta B-11(melt-Tur) = 0.90 +/- 0.05 parts per thousand at 660 degrees C and +0.23 +/- 0.12 parts per thousand at 800 degrees C), and the temperature dependence can be defined as Delta B-11(melt-Tur) = 4.51 x (1000/T [K]) - 3.94 (R-2 = 0.96). Using these boron isotope fractionation factors, tourmaline can serve as a tracer to quantitatively interpret boron isotopic ratios in evolved magmatic systems. Our observation that B-11 is enriched in granitic melt relative to tourmaline suggests that the delta B-11 of late-magmatic tourmaline should be higher than tourmaline that ciystallised at an early stage, if B isotope fractionation is not affected by other processes, such as fluid loss.