Boron isotope composition of detrital tourmaline: A new tool in provenance analysis

Tourmaline is a borosilicate mineral commonly found in sedimentary rocks worldwide. It is chemically and mechanically stable and, as a host of boron in crustal rocks, it could be a useful indicator of rock sources. Despite this, its potential as a source indicator using the boron (B) isotope has remained largely under-explored. In this study, we propose a new integrated approach to fingerprint the origin of detrital tourmaline grains and test whether the approach is useful for identifying source rocks. Tourmaline grains in modern river sands from Southern Tibet were analyzed for optical properties, major elements, and B isotope composition. The geochemistry of detrital tourmaline in sand from the Nianchu and Pumchu rivers draining the Himalayan ranges is similar, and the B isotope values indicated they were mainly from Himalayan leucogranites. Tourmalines in the Lhasa River sand were derived from two major sources. Grains with high Al, low Mg, and strongly negative B isotope values (−12‰ to −16‰) were interpreted from the Gangdese arc batholith, whereas those with low Al, high Fe, and B isotope values of −2‰ to 2‰ were proposed most likely from Nyaingentanglha orthogneisses. We examined the relationships between tourmaline properties and found that B isotope values are useful as a source indicator. We also established a database of B isotope values for 2724 tourmaline crystals in different types of exposed rock spanning 37 countries, to promote the use of B isotope values in tourmaline in provenance studies.