Find in Library

We determined the bromine isotope compositions of magmatic and hydrothermal sodalite (Na₈Al₆Si₆O₂₄Cl₂) and tugtupite (Na₈Al₂Be₂Si₈O₂₄Cl₂) from the Ilímaussaq intrusion in South Greenland, in order to constrain the Br isotope composition of the melt and hydrothermal fluids from which these minerals were formed. Early formed magmatic sodalite has high Br contents (138 ± 10 µg/g, n = 5) and low δ⁸¹Br values (+0.23 ± 0.07‰). Late stage hydrothermal sodalite has lower Br contents (53±10 µg/g, n = 5) and higher δ⁸¹Br values (+0.36 ± 0.08‰). Tugtupite that forms at even later stages shows the lowest Br contents (26 ± 2 µg/g, n = 2) and the highest δ⁸¹Br values (+0.71 ± 0.17‰). One hydrothermal sodalite has a Br concentration of 48 ± 9 µg/g and an exceptionally high δ⁸¹Br of 0.82 ± 0.12‰, very similar to the δ⁸¹Br of tugtupites. We suggest that this may be a very late stage sodalite that possibly formed under Be deficient conditions. The data set suggests that sodalite crystallises with a negative Br isotope fractionation factor, which means that the sodalite has a more negative δ⁸¹Br than the melt, of −0.3 to −0.4‰ from the melt. This leads to a value of +0.5 to +0.6‰ relative to SMOB for the melt from which sodalite crystallises. This value is similar to a recently published δ⁸¹Br value of +0.7‰ for very deep geothermal fluids with very high R/Ra He isotope ratios, presumably derived from the mantle. During crystallisation of later stage hydrothermal sodalite and the Be mineral tugtupite, δ⁸¹Br of the residual fluids (both melt and hydrothermal fluid) increases as light ⁷⁹Br crystallises in the sodalite and tugtupite. This results in increasing δ⁸¹Br values of later stage minerals that crystallise with comparable fractionation factors from a fluid with increasingly higher δ⁸¹Br values.