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Abstract

Tourmaline is the most abundant borosilicate in the Earth’s crust and the ratio of its two B isotopes (¹¹B/¹⁰B) helps to unravel the source and evolution of fluids during metamorphic, magmatic and hydrothermal processes. For example, tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch area in the western TauernWindow (Alps). Here, tourmaline crystals show successively decreasing B isotope ratios from their cores to their rims. In combination with whole rock B isotope data and a Rayleigh fractionation model it is shown that the B-isotope signatures in tourmaline are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline. The reliable interpretation of B isotope patterns in natural tourmaline requires knowledge about the B isotope fractionation between tourmaline and fluid (∆¹¹Btur-fluid). Up to date, experimental studies only considered tourmaline with B in trigonal coordination ([3]B), but many natural tourmalines incorporate excess B at the tetrahedrally coordinated T site ([4]B). In this study, ∆¹¹Btur-fluid values are determined as a function of [4]B-content in tourmaline based on tourmaline syntheses experiments at high pressures of 4.0 GPa and temperatures of 700°C using a piston-cylinder apparatus. Olenitic tourmaline rich in [4]B [∼ 2.5 atoms per formula unit (pfu)] has been synthesized in the system SiO₂-Al₂O₃-B₂O₃-NaCl-H₂O. Applying time dependent experiments it is shown that these form via a non-classical crystallization pathway involving jeremejevite as a precursor phase. Rossmanitic tourmaline with lower amounts of [4]B (∼ 0.6 pfu) has been synthesized in the system Li₂O-SiO₂-Al₂O₃-B₂O₃-H₂O. The mechanism for the incorporation of B at the T site follows TSi-1 VO-1 TB1 V(OH)1 as the main and X -1 TSi-1 XNa1 TB1 as a minor exchange vector. Raman spectroscopy has been successfully applied for the chemical characterization of synthetic tourmaline, including the quantification of [4]B concentrations and the first-time detection of Li at the X site. To derive ∆¹¹Btur-fluid as a function of [4]B, B isotope ratios of the synthetic olenitic tourmaline have been determined by spatially resolved secondary ion mass spectroscopy (SIMS), whereas the B isotope ratios of coexisting fluids have been measured via multi-collector plasma source mass spectroscopy. Most accurate SIMS data were obtained using a natural olenite from the Koralpe as a novel reference material, pointing towards the need of matrix matched reference material for SIMS B-isotope analysis of tourmaline. The results showthat if 10 mol% of the total B occur in tetrahedral coordination, the ∆¹¹Btur-fluid is shifted by 0.8 ± 0.5% at 700°C towards more negative values. This corresponds to an intracrystalline fractionation of 8 ± 5% , whereby ¹⁰B preferentially occupies the tetrahedral T site relative to the trigonal B site. Possibly, the effect of tetrahedral boron in tourmaline on the B-isotope fractionation is even greater at lower temperatures.