Boron isotope and light element sector zoning in tourmaline: Implications for the formation of B-isotopic signatures

Abstract

Hourglass sector zoning in metamorphic tourmaline is known to effectively fractionate the major and trace elements between different sectors, producing three distinct compositions at the growth surface. Here we show that the light elements Li, Be, H and B, as well as the δ 11B signature are also affected by sector zoning. The sector enrichment in elements is controlled by a dual process of initial preference due to growth surface charge and morphology, followed by consecutive, charge-balance controlled enrichment. The light elements, especially B and H, appear to act as charge-balance cations in this mechanism, with a preferred B 3+ for Si 4+ and O 2− for OH − substitution in the c − sector. A lighter δ 11B signature ( Δ c + c − = 1.8 ± 0.6 ‰ ) accompanies this increase in B IV in the c − sector, in line with the lighter signatures for B IV compared to B III in minerals. However, it exceeds this theoretical fractionation by a factor of 10. Although we have not identified a definite process that is responsible for this fractionation, it may be related to local variations in host medium along the growth surface resulting from preferred uptake of elements. These observations show that δ 11B signatures in tourmaline are not an independent measure of its host environment, but also a function of mineral composition, and the 3-dimensional zoning variations herein.