The geochemistry of Tl and its isotopes during magmatic and hydrothermal processes: The peralkaline Ilimaussaq complex, southwest Greenland

• Published in: Chemical geology Vol. 366; pp. 1 - 13
• Main Authors: Hettmann, Kai, Marks, Michael A.W., Kreissig, Katharina, Zack, Thomas, Wenzel, Thomas, Rehkämper, Mark, Jacob, Dorrit E., Markl, Gregor
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 14-02-2014
• Subjects: Alkalinity; Arctic; Assimilation; Astrophyllite; Crustal materials; Degassing; Differentiation; Djerfisherite; Enrichment; Geochemistry; Geokemi; Greenland; Hydrothermal process; Ilimaussaq Complex; Isotopes; Isotopic composition; LILE; magma assimilation; magmatic differentiation; Magmatic process; Magmatic processes; magmatism; Minerals; Ores; Peralkaline; peralkaline rock; Precipitation; Silicate minerals; Thallium; Thallium isotopes; AN, Greenland; Thallium isotopes; Magmatic processes; Peralkaline; LILE
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2013.12.004

We use thallium (Tl) concentrations, K/Rb, K/Tl and Rb/Tl ratios and Tl isotopes in minerals from the alkaline to peralkaline Ilimaussaq complex (South Greenland) to trace magmatic differentiation, crustal assimilation, magmatic degassing, ore precipitation and hydrothermal metasomatism. Closed-system magmatic differentiation is marked by a coherent decrease of K/Tl- and K/Rb-ratios, whereas crustal assimilation results in a strong Tl-enrichment, causing low K/Tl-ratios compared to K/Rb-ratios. Thallium isotopes show only slight changes during orthomagmatic differentiation and the assimilation of crustal material cannot be traced, since the isotopic composition of the average crust is within the range of the mantle and mantle-derived rocks. Magmatic degassing, however, increases Rb/Tl-ratios and changes the isotopic composition of Tl. The released fluids are enriched in Tl, characterized by high 205Tl/203Tl ratios and can precipitate Tl-rich sulfide and silicate minerals as indicated by some late-magmatic hydrothermal veins, which contain a conspicuous assemblage of Tl–Fe–Cu-sulfides (thalcusite, djerfisherite, chalcothallite). The oxidative alteration of these assemblages at high pH results in small-scale redistribution of Tl. Thallium released by this process is entrained into late-stage Tl-enriched astrophyllite.
•Tl and its isotopes is a good geochemical tracer for magmatic and hydrothermal processes.•Tl behaves similar to Rb during magmatic processes and incorporates into silicates.•Tl fractionates from Rb during hydrothermal stage and incorporates into sulfides.•Tl isotopes are fractionated even under high-T magmatic conditions.•Tl is redistributed from sulfides to silicates during alteration.