Boron isotopic composition of zoned (schorl-elbaite) tourmalines, Mt. Capanne Li-Cs pegmatites, Elba (Italy)
Tonarini, Sonia; Dini, Andrea; Pezzotta, Federico; Leeman, William P.
European Journal of Mineralogy Volume 10 Number 5 (1998), p. 941 - 952
published: Oct 5, 1998
manuscript accepted: Apr 1, 1998
manuscript received: Jul 21, 1997
ArtNo. ESP147051005009, Price: 29.00 €
Abstract 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 °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 (δ11B from -9.6 to -8.5 %c) 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 Δfluid-tour = 0 (i.e., no isotopic fractionation occurs). In the second case, "melt/fluid" composition was increasingly enriched in 11B 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 in11B compared to associated crystalline assemblages.