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

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-syst...

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Bibliographic Details
Published in:Chemical Geology
Main Authors: Hettmann, Kai, Marks, Michael A W, Kreissig, Katharina, Zack, Thomas, Wenzel, Thomas, Rehkämper, Mark, Jacob, Dorrit E., Markl, Gregor
Format: Article in Journal/Newspaper
Language:English
Published: 2014
Subjects:
LILE
Magmatic processes
Peralkaline
Thallium isotopes
Greenland
Ilimaussaq
Online Access:https://researchers.mq.edu.au/en/publications/2910659d-cf1c-44c3-921c-da69a5d35ad4
https://doi.org/10.1016/j.chemgeo.2013.12.004
http://www.scopus.com/inward/record.url?scp=84891823182&partnerID=8YFLogxK
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Summary: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 205 Tl/ 203 Tl 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.

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