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 delta 11 B values of tourmalines from granites, pegmatites, and associated hydrothermal veins (-29.9 per mil to +8.6 per mil). The boron isotopic compositions of granitic tourmalines are 11 B 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 delta 11 B values with a maximum at -10 per mil to -5 per mil. 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 delta 11 B values of the magma. During magma degassing 11 B is preferentially partitioned into the vapour phase, leading to lower delta 11 B values in magmatic tourmalines relative to the exsolved hydrothermal fluids. Degassing also leads to a decrease in delta 11 B values from the early to late magmatic tourmalines.