The boron isotopic evolution of the Little Three pegmatites, Ramona, CA

Interpretations of δ11BTur from margin-to-core in these and other pegmatites are hampered by uncertainties in the boron isotopic fractionation factors for melt-aqueous fluid and for tourmaline-granitic melt. Based on modeled cooling and crystallization of these dikes, the growth of tourmaline should have been fast relative to the diffusivity of B through granitic melt. Hence, an equilibrium distribution of 11B and 10B between tourmaline, bulk melt, and an aqueous solution (i.e. Rayleigh fractionation process) is unlikely. The negligible isotopic fractionation of boron in tourmaline from margins to center, therefore, probably records only the bulk isotopic composition of the pegmatite-forming melts.