Boron isotopic composition of zoned (schorl-elbaite) tourmalines, Mt. Capanne Li-Cs pegmatites, Elba (Italy)

Boron isotopic compositions are found to be nearly uniform within progressive growth sectors of three zoned tourmaline samples, each from a different miarolitic pegmatite on Elba Island. Chemistry (schorl to elbaite) and textures of these samples indicate that they formed from an evolving medium that varied from melt, through melt + vapor, to vapor only assemblages (here designated simply as "melt/fluid") with decreasing temperature (ca. 650-300 degrees C). Closed-system crystallization conditions are inferred in two cases whereas, in one case, late fracturing and open-system conditions resulted in development of fibrous schorl overgrowths. The negligible variation in tourmaline isotopic composition (delta(11)B from -9.6 to -8.5 parts per thousand) is tentatively explained in two different ways. In the first case, the isotopic fractionation factor between tourmaline and either fluid-unsaturated melt, fluid-saturated melt, or aqueous fluids is near-constant near Delta(fluid-tour) = 0 (i.e., no isotopic fractionation occurs). In the second case, "melt/fluid" composition was increasingly enriched in B-11 due to increasing "melt/fluid"-tourmaline isotopic fractionation as temperature dropped. The latter scenario is more consistent with the few presently available experimental data on tourmaline-fluid isotopic fractionation at high temperatures, and implies that evolving magmatic fluids will be relatively enriched in B-11 compared to associated crystalline assemblages.