Changes in Halogen (F, Cl, Br, and I) and S Ratios in Rock-Forming Minerals as Monitors for Magmatic Differentiation, Volatile-Loss, and Hydrothermal Overprint: The Case for Peralkaline Systems

Abstract

We determined the halogen (F, Cl, Br, and I) and sulfur (S) concentrations in Cl-rich rock-forming minerals from five peralkaline complexes. We investigated sodalite (N = 42), eudialyte-group minerals (N = 84), and tugtupite (N = 8) from representative rock samples derived from Ilímaussaq (South Greenland), Norra Kärr (Sweden), Tamazeght (Morocco), Lovozero, and Khibina (Russian Federation). Taken together, sodalite and eudialyte-group minerals dominate the Cl and Br budget of the investigated rocks. For F, however, several other phases (e.g., amphibole, fluorite, villiaumite, and minerals of the rinkite group and the apatite supergroup) are additional sinks, and parts of the S may be scavenged in generally rare sulfides. The investigated minerals contain Cl at the wt.% level, F and S concentrations are in the hundreds to thousands of µg/g-range, Br is less common (0.2–200 µg/g) and I is rare (mostly well below 1 µg/g). Normalized to Cl, sodalite prefers Br relative to eudialyte-group minerals, while F is always enriched in the latter. Our data show that both F and S may represent important components in eudialyte-group minerals, sometimes at similar levels as Cl, which normally dominates. Sulfur reveals redox-dependent behavior: Under reduced crystallization conditions, S is more compatible in eudialyte-group minerals (EGM) than in sodalite, which flips to the opposite under water-rich and presumably more oxidized conditions. We investigate the applicability of F/Cl, Br/Cl, and S/Cl ratios in these minerals in peralkaline systems to better understand the interplay of magmatic differentiation, fluid loss and hydrothermal overprint. Similar to apatite in metaluminous systems, fractionation of sodalite, and eudialyte-group minerals in peralkaline magmas leads to decreasing Br/Cl ratios. The data presented in this study bear implications for the mineral chemistry and compositional variation of sodalite and especially EGM in general. Volatile components in EGM that are not normally considered, such as F and S, can reach concentrations of thousands of µg/g. Especially in the case of F, with its low atomic weight, the results obtained in this study indicate that it is very significant for formulae calculations, neutral charge-balance, and similar aspects at such concentration levels. This study demonstrates that halogen contents and ratios are sensitive monitors for a variety of processes in magmatic-hydrothermal systems, including magmatic fractionation, volatile loss, and fluid–rock interaction.

Bromine Isotope Variations in Magmatic and Hydrothermal Sodalite and Tugtupite and the Estimation of Br Isotope Fractionation between Melt and Sodalite

“…δ 81 Br values of the magmatic sodalites in our sample set are between 0.15 and 0.31‰ with an average value of 0.23 ± 0.07‰. Magmatic sodalite in the Ilímaussaq intrusion crystallised from the final differentiation stage of the magma [21].…”

“…These differences are probably an extraction artefact. As the analytical data obtained by the two extraction methods are comparable, we use the data published in our recent study [21] in the discussions throughout, to maintain consistency between the two studies.…”

“…These two mineral groups host most Cl and Br that was present in the melt, as the two elements show very similar behaviour in magmatic processes [24]. Importantly, SGMs take up Br preferentially compared to EGMs [21]. It is assumed that Br concentrates in the final melt from which SGMs (in principle sodalite proper) crystallise, as it is generally assumed that little Br is removed from the melt before sodalite crystallises.…”

“…Samples used in this study derive from the Ilímaussaq Intrusion (South Greenland), which is one of the most evolved peralkaline intrusions known and contains large quantities of the Cl rich minerals sodalite and eudialyte [22]. For this study, we used previously characterized magmatic and hydrothermal sodalite and hydrothermal tugtupite samples [21]. Magmatic sodalites derive from coarse-grained plutonic rocks that contain abundant euhedral sodalite (up to 70 vol.%) and interstitial alkali feldspar, nepheline, amphibole, and eudialyte.…”

“…A structurally similar mineral (tugtupite, Na 8 Be 2 Al 2 Si 8 O 24 Cl 2 ) crystallises under hydrothermal condi-tions only. The Br contents in these minerals vary between 25 and 150 µg/g [21]. Thus, 0.15 to 1 g of mineral separate is sufficient for Br isotope analysis [5], so that these minerals are a valid target for the determination of Br isotopes in magmatic rocks and hydrothermal systems.…”