The diversity of B-isotope variations in tourmaline from rare-element pegmatites in the Borborema Province of Brazil

Abstract

Tourmaline chemical and B-isotope compositions have been determined from 4 rare-element pegmatites and 1 barren pegmatite in the Borborema Pegmatite Province of NE Brazil, and also from a leucogranite considered to be parental to the pegmatites. The rare-element pegmatites are of the LCT (lithium–cesium–tantalum) class and they display a classic zoning structure with tourmaline present in all units but the quartz core. Tourmaline major-element composition varies systematically from schorl–dravite in the border and wall zones to varicolored elbaite in the inner zones, consistent with increasing differentiation of a pegmatite magma. Previous studies supposed a continuous variation in Li contents from schorl–dravite to elbaite but the SIMS data instead show a major gap between 0.07wt.% and 0.72wt.% Li2O, followed by a continuous variation to the maximum value of 1.8wt.%. The barren pegmatite and leucogranite contain schorl–dravite only.The full range of δ11B values in tourmaline from the Borborema pegmatites is from −20.2 to +1.6‰ (n=62), which comes close to the total known diversity of pegmatite tourmalines worldwide and is much broader than the range of the associated leucogranite (−15.1 to −13.4‰, n=3). However, more than 80% of the pegmatite tourmalines define a “main range” between −17 and −9‰ δ11B. The average value of the latter (−13.5‰, s.d.=2.3‰, n=50) is identical within error with the leucogranite, supporting suggestions of a genetic relationship. The B-isotope compositions of barren and mineralized pegmatites overlap within the main range. We find no correlation between chemical composition and B-isotope ratios. Famous for the Borboreman province are gem-quality crystals of blue “Paraíba type” elbaite. The δ11B values of blue tourmaline from the Capoeira pegmatite also fall within the main range (−16.4 to −11.9‰, n=5).Four of the 5 pegmatites studied are heterogeneous in δ11B, with typical variations of 6‰. However, there are no consistent trends in these examples from outer to inner units or from core to rim of zoned crystals that would fit any one scenario of melt evolution. Instead, the factors controlling B-isotope evolution, e.g., crystallization, fluid exsolution or mixing with external B reservoirs, appear to vary locally and can be specific to the individual pegmatite or internal zone within it. For example, the Carrascão pegmatite shows a shift from higher to lower δ11B values in tourmaline from the outer to inner zone (−14.9 to −9.1‰ vs. −20.2 to −13.9‰, resp.), and there is a parallel trend of lighter rims in zoned elbaite from the inner intemediate zone. These trends are consistent with 11B removal by tourmaline crystallization and/or fluid exsolution. An opposite trend in isotopic variation was found in zoned tourmaline from the wall zone of the Boqueirão pegmatite, whose core compositions are in the main range (−15.1 to −14.9‰) and whose rims are much heavier (−5.5 to −1.7‰). Similarly high δ11B values, about 10‰ beyond the main range, were found in wall-zone tourmaline in the barren Fazenda Turmalina pegmatite (−6.0 to +1.6‰). The patchy distribution of heavy B-isotopic compositions, found only in outer zones of pegmatites, and the strong isotopic contrast with the main range suggest the possibility of mixing with heavy B from the country rocks, a potential source being layers of marble and calc-silicate rocks.