Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, B]_fluid/B]_tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. ¹¹B always preferentially fractionated into the fluid. For experiments where B]_fluid/B]_tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ¹¹B_{(tour–fluid)} = −4.20 · 1,000/T (K)] + 3.52; R ² = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where B]_fluid/B]_tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.