Experiments reveal enrichment of 11B in granitic melt resulting from tourmaline crystallisation
Lin Cheng, Chao Zhang, Yang-Zhang Zhou, I. Horn, Stefan Weyer, François Holtz
Abstract
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 (B 2 O 3 8 wt. %) at 660-800 C, 300 MPa, and a H 2 O = 1, in which tourmaline occurs as the only boron-hosting mineral. Our experimental results at four different temperatures show a small and temperaturedependent boron isotope fractionation between granitic melt and tourmaline ( 11 B melt-Tur = 0.90 0.05 at 660 C and 0.23 0.12 at 800 C), and the temperature dependence can be defined as 11 B melt-Tur = 4.51 (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 11 B is enriched in granitic melt relative to tourmaline suggests that the 11 B of late-magmatic tourmaline should be higher than tourmaline that crystallised at an early stage, if B isotope fractionation is not affected by other processes, such as fluid loss.