Abstract
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.
Highlights
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Licensee MDPI, Basel, Switzerland.Since the development of techniques to measure stable bromine (Br) isotopes in geological systems [1], our knowledge on Br isotope systematics has increased significantly.Our knowledge is currently mostly restricted to aqueous systems and salt deposits and to some environmentally relevant organobromine systems that are Br-rich, and where Br can be extracted in sufficient quantities for isotope analyses [2]
Br concentrations and δ81 Br values change systematically between the three sample types (Table 1, Figure 1). δ81 Br values are lowest in magmatic sodalite samples, slightly higher in hydrothermal sodalite samples, and significantly higher in the tugtupite samples
Br concentrations are highest in the magmatic sodalite samples and are lower in hydrothermal sodalite samples, and even lower in tugtupite samples
Summary
Since the development of techniques to measure stable bromine (Br) isotopes in geological systems [1], our knowledge on Br isotope systematics has increased significantly. Our knowledge is currently mostly restricted to aqueous systems and salt deposits and to some environmentally relevant organobromine systems that are Br-rich, and where Br can be extracted in sufficient quantities for isotope analyses [2]. Depending on the analytical technique either about 1 mg (isotope ratio mass spectrometry (IRMS) [1,3]) or 25 μg (multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) [4,5]). Of Br is necessary for a reliable measurement. The large amount of Br necessary for isotope analysis has, until now, prevented the measurement of Br isotopes in rock samples
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