The boron isotopic composition of tourmaline as a guide to fluid processes in the southwestern England orefield: An ion microprobe study

Abstract

The boron isotopic composition ( δ 11B) and chemistry of tourmalines from a series of sites in the western southwest England orefield has been determined using ion microprobe and electron microprobe techniques, respectively. The range of δ 11B values determined is relatively wide (+0.6 to −12.7‰) and falls at the lighter end of the range for typical continental crust. Within this range granite tourmalines show a depletion in 11B from coarse grained early magmatic examples to fine-grained late magmatic tourmaline. We attribute this to fractionation of boron isotopes between aqueous and melt species during the exsolution of an aqueous fluid phase from the magma. The aqueous phase formed by degassing of the magma is considered to have crystallised to form the tourmalinites and quartz-topaztourmaline rocks common throughout the area. Decreases in the δ 11B values occur within certain zones of individual tourmaline crystals from these rocks. This is ascribed to loss of 11B in the vapour phase during boiling caused by decompression of the aqueous fluid on fracturing. The calculated boron isotope composition of fluids in equilibrium with tourmalines from the Cligga Head greisen bordered vein system agrees closely with that calculated for the tourmalinite fluid and suggests a constant, granite-derived source of boron during the early hydrothermal evolution of the stock. The lack of isotopic zoning within the tourmalines from the Cligga Head hydrothermal veins suggests that phase separation did not occur or was very limited during hydrothermal circulation in the vein system.