An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid

摘要

This study investigates the effect of tetrahedral B (B-[4]) in synthetic tourmaline on the B-isotope fractionation between tourmaline and fluid. This is important for the correct interpretation of B-isotope variations in natural tourmalines containing "excess" B (greater than three atoms per formula unit), which substitutes for Si at tetrahedral sites. Such tourmalines commonly occur in Li, Al-rich pegmatites and have been reported from glaucophane schists that formed at high pressures during subduction. Tourmaline synthesis experiments were performed in a piston-cylinder apparatus in the system SiO2-Al2O3-B2O3-NaCl-H2O at 4 GPa and 700 degrees C using different run durations, starting from quartz-gamma-Al2O3-H3BO3 solid mixtures and NaCl-solutions. We were able to produce "olenitic" tourmaline with excess B between 1.2 and 2.5 B-[4] per formula unit. The B-isotope compositions of the olenitic tourmaline and coexisting fluids were determined by secondary ion mass spectrometry and multi-collector plasma source mass spectrometry to derive isotope fractionation coefficients. The results indicate that for every 10 mol% of total B in tourmaline in tetrahedral coordination, the value of Delta B-11(tur-fluid) is shifted to more negative values by about 1 parts per thousand at 700 degrees C. This is in good agreement with published ab initio calculations and corresponds to an intracrystalline fractionation of B-isotopes between the trigonal B and tetrahedral T sites of tourmaline on the order of 8 +/- 5 parts per thousand, whereby B-10 partitions to the T site.