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

Abstract

Tourmaline, mica, hambergite, danburite, and axinite from three pegmatite dikes (named Main, Swamp, and Spessartine) at the Little Three mine near Ramona, California, have been analyzed by secondary ion mass spectrometry (SIMS) for their boron isotopic compositions (δ11B). Tourmaline is the sole host of B in the massive footwall and hanging wall sections of the dikes, and the principal sink for B in the miarolitic cavities. In the thinner Spessartine and Swamp dikes, the pocket mineralogy includes foitite, spessartine, muscovite, axinite, danburite, and rare beryl. The thicker Main dike culminates in cavities containing topaz, elbaite, lepidolite, boromuscovite, hambergite, and rare stibiotantalite.Values of δ11BTur from the Little Three pegmatites are strikingly heavy (−0.1‰ to +13.8‰) compared to other pegmatitic tourmaline (~−27 to +9‰), which is best explained by inheritance from an 11B-enriched source material such as altered oceanic crust. Flat patterns of δ11BTur from margin-to-core of each pegmatite attest to a negligible fractionation of boron isotopes between tourmaline-granitic melt and granitic melt-aqueous fluid. Tourmaline samples from intermediate zones and miarolitic cavities have identical boron isotopic compositions, except for miarolitic cavities that contain axinite and danburite, whose B is in 4-fold structural coordination. In these cavities, the δ11B of tourmaline increases relative to tourmaline from the intermediate zone.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.