Fractionation of the isotopes of boron between granitic melt and aqueous solution at 700 °C and 800 °C (200 MPa)

The fractionation of boron's isotopes, 10B and 11B, between granitic melt (M) and aqueous solution (V), represented as Δ11BM-V, was investigated between 700 °C and 800 °C at 200 MPaH2O. One series of experiments used the Macusani obsidian (MAC) to reassess the Δ11BM-V values reported by Hervig et al. (2002). Another series of experiments employed a synthetic glass (HG) made to the composition of the hydrous haplogranite minimum (Ab36Or29Qtz35) at 200 MPaH2O. Boron was added as boric acid (NIST SRM 951a) to two aliquots of HG glass to bring the concentration of B2O3 in each to ~2.5 (‘Low B’) and ~5 wt% B2O3 (‘High B’). Thus, values of Δ11BM-V were examined as a function of B2O3 and temperature. Results from the current study show significant fractionation of boron's isotopes between granitic melt and aqueous solution at these high temperatures. Values of Δ11BM-V for ‘Low B’ and ‘High B’ glasses range from −10.9 (±1.3‰) to −6.1 (±1.7‰) at 800 °C and −7.8 (±1.5‰) to −5.3 (±1.3‰) at 700 °C, and the Δ11BM-V values do not correlate with the concentration of B in melt (glass). One experiment (CGB88) conducted at 750 °C and 200 MPaH2O using MAC yielded a Δ11BM-V value (−8.4‰) similar to that reported by Hervig et al. (2002). The Δ11BM-V values reported in this study form a linear trend with temperature through the Δ11B values for boromuscovite-aqueous solution, illite-aqueous solution, and melt-aqueous solution. Therefore, based on the results of this study, we concur with Hervig et al. (2002) that the fraction of B in 4-fold coordination in hydrous granitic melt is significant, if this is the fundamental cause of the fractionation effect. Spectroscopic studies on hydrous glass compositions similar to those reported in this study might elucidate the coordination environment of B in melts to the extent that its structural state is quenchable.