EXPERIMENTAL-DETERMINATION OF FRACTIONATION OF B-11/B-10 BETWEEN TOURMALINE AND AQUEOUS VAPOR - A TEMPERATURE-DEPENDENT AND PRESSURE-DEPENDENT ISOTOPIC SYSTEM

The extent of boron isotope fractionation between tourmaline and aqueous vapor has been determined over the P-T range 50-200 MPa and 350-750-degrees-C. The results show that the extent of fractionation is inversely dependent on temperature. An unusual feature of the reaction is that the isotope fractionation is less extensive at higher pressures for an equivalent temperature. Due to its refractory nature the experiments involved synthesis of tourmaline from constituents rather than equilibration of the preformed mineral with an isotopically distinct aqueous phase. For this reason we cannot, as yet, be certain whether the pressure effect is the result of equilibrium or kinetic processes.