THE BORON ISOTOPIC COMPOSITION OF TOURMALINE FROM PEGMATITES AT THE LITTLE THREE MINE, RAMONA, CALIFORNIA

Tourmaline, mica, hambergite, danburite, and axinite, from the Main, Swamp, and Spessartine pegmatite dikes 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 primary host of B in the footwall and hanging wall sections of the dikes. 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. The values of δ¹¹B_Tur are strikingly heavy (-0.6 to +9.7‰) compared to most other pegmatitic tourmaline, which may reflect inheritance from altered oceanic crust with or without a marine sediment component. Across the footwall aplite of the Swamp dike, values of δ¹¹B_Tur are constant up to the lower intermediate zone, at which point the values abruptly decrease (~2‰). The footwall aplite of the Main dike, which can be divided into massive and layered sections, displays small, 2-4‰, fluctuations in δ¹¹B_Tur. There is a symmetrical decrease in δ¹¹B_Tur from the upper and lower intermediate zones into miarolitic cavities of the Main and Swamp dikes. The difference in δ¹¹B between tourmaline from the intermediate zones and miarolitic cavities in the Main and Spessartine dikes is nil. The δ¹¹B of tourmaline increases sharply from intermediate zone to miarolitic cavity where danburite and axinite (both with 4-coordinate B) co-crystallized.

Uncertainties in the boron isotopic fractionation factors for melt-aqueous fluid and tourmaline-granitic melt hamper interpretations of magmatic values of δ¹¹B_Tur. However, considering the growth rate of tourmaline should have been fast relative to the diffusivity of B through granitic melt, an equilibrium distribution of ¹¹B and ¹⁰B between tourmaline, bulk melt, and an aqueous fluid (i.e. Rayleigh fractionation process) is unlikely. The boron isotopic composition of aqueous fluid present during the formation of miarolitic cavities and the tourmalinized host tonalite can be reconciled to equilibria using boron isotopic fractionation factors between tourmaline and aqueous fluid insofar as these are known.