Boron isotope systematics of tourmaline from granites and pegmatites: a synthesis

Abstract Tourmaline is abundant in many granites and pegmatites and is generally the most significant mineral sink for boron crystallising from a silicic melt. There is a wide variation in the d11B values of tourmalines from granites, pegmatites, and associated hydrothermal veins (-29.9 %c to +8.6 %o). The boron isotopic compositions of granitic tourmalines are < 0 %o and can be as low as -30 %o, but most of the data cluster between -15 %o and 5 %o. A similar d11B distribution is seen in pegmatitic tourmalines, but tourmalines from granite-related veins at granite-country rock contacts or within adjacent country rocks show generally higher d11B values with a maximum at -10 %o to -5 %o. The boron isotopic compositions of tourmaline from these settings are largely controlled by the composition of the magma source, magma degassing, contributions of boron from country rocks, and P-T conditions of tourmaline formation. The pre-melting history of boron mobilisation and its redistribution in granite source rocks may also have pronounced effects on boron concentrations and d11B values of the magma. During magma degassing d11B is preferentially partitioned into the vapour phase, leading to lower d11B values in magmatic tourmalines relative to the exsolved hydrothermal fluids. Degassing also leads to a decrease in d11B values from the early to late magmatic tourmalines