Boron isotope geochemistry: new insights from the study of borates and boro-silicates in granite-pegmatite systems

Recently, the isotopic composition of boron has revealed itself an innovative geochemical tracer in studies of ore-genesis and petrogenesis. Tourmaline represent the most important mineralogical sink for boron in granite-pegmatite systems but other phases such as danburite, hambergite, rhodizite sometimes can play a significant role. Petrogenetic application of boron isotopes to granites and pegmatites is still partially restricted by the poor knowledge of boron isotope fractionation processes, occurring in these systems during the crystallization of boron minerals. In order to understand the behavior of boron and its isotope fractionation during the evolution of granite-pegmatite systems, d11B was measured in three- and four-coordinated boron minerals from two natural cases: 1) tourmaline-dominated LCT pegmatites (Elba Island, Italy) representative of most widespread pegmatite systems; 2) uncommon pegmatites of Madagascar, with three- and four-coordinated boron minerals (toumaline, hambergite, danburite and rhodizite). At Elba Island,??11B of late magmatic schorl from granitic host rocks (-8.3/-9.6?), early schorl in massive zone of pegmatites (-8.5/-9.4?), and late, zoned (schorl-elbaite), crystals from miarolitic cavities (-8.6/-9.6?), point to the absence of boron isotopic fractionation during crystallization, from melt and aqueous fluids, with temperature decrease. On the contrary, in the Anjanabonoina pegmatite (Madagascar) significant d11B variations were observed between tourmalines coming from early, danburite-rich, massive zones (+4.5?), and late, danburite-free, miarolitic zones (+6.0/7.0?; +14.0? in a single pocket). In the former zone, danburite shows a significant, negative, shift in d11B in respect to the associated tourmalines (?tour-danb=5.5?). Cogenetic tourmaline and rhodizite from another locality in Madagascar (Tetezantsio) showed a similar shift in d11B. Finally, it is remarkable that d11B of continuously zoned (dravite-elbaite) crystals from the late, danburite-free, miarolitic cavities do not vary with the pocket evolution. Constant d11B data on tourmaline from the Elba granite-pegmatite, peraluminous, system suggest that boron in melt and in aqueous fluids was trigonally coordinated. Then, in simple systems dominated by trigonal boron, both in minerals and in the growing medium(s), the d11B of tourmaline can approach the isotopic signature of the melt. On the other hand, crystallization of large amount of four-coordinated boron minerals (danburite and/or rhodizite), during the early massive stage of complex pegmatites (Madagascar), produced an enrichment of 11B in the residual fluids from which miarolitic tourmalines crystallized. In spite of the complex evolution of boron isotopic composition in such uncommon pegmatite, the collected data can help us to shed new lights on speciation, and isotope fractionation of boron in granite-pegmatite systems.