BORON ISOTOPIC COMPOSITION OF ZONED (SCHORL-ELBAITE) TOURMALINES, MT.CAPANNE LI-CS PEGMATITES, ELBA (ITALY)

Baron isotopic compositions are found to be nearly uniform within prog ressive growth sectors of three zoned tourmaline samples, each from a different miarolitic pegmatite on Elba Island. Chemistry (schorl to el baite) 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 decr easing temperature (ca. 650-300 degrees C). Closed-system crystallizat ion conditions are inferred in two cases whereas, in one case, late fr acturing and open-system conditions resulted in development of fibrous schorl overgrowths. The negligible variation in tourmaline isotopic c omposition (delta(11)B from -9.6 to -8.5 parts per thousand) is tentat ively 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 De lta(fluid-tour) = 0 (i.e., no isotopic fractionation occurs). In the s econd 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 th e few presently available experimental data on tourmaline-fluid isotop ic fractionation at high temperatures, and implies that evolving magma tic fluids will be relatively enriched in B-11 compared to associated crystalline assemblages.