Fluorite (U-Th)/He geochronology: methodical development and case studies
Final Report Abstract
We extended the scope of the first studies using fluorite (U-Th-Sm)/He geothermometer (FHe) of Pi et al. (2005) and Evans et al. (2005) and performed for the first time a systematic investigation of the diffusion of He in fluorite: A test sample collection of fluorites was compiled covering a wide range of colour, composition and genetic characteristics. - The analytical background was prepared and calibrated (He diffusion and multi-element analysis on microgram sized aliquots). - The helium diffusion experiments yielded reproducible results. - We detected that the heating schedule of the laboratory procedure affects the results. This may have consequences in the evaluation of diffusion data generated in different laboratories. - The different fluorite samples yielded highly variable diffusion parameters and the calculated closure temperatures cover a wide range between 43 ± 14 °C and 170 ± 9 °C. - Diffusion experiments performed on different grain sizes from the same sample showed that the diffusion domain is bigger than 125 µm, and supposedly approximates the physical size of the crystals. This should be considered when interpreting FHe ages. - The closure temperature shows correlation to the total REE content of the fluorite. This can be explained by narrowing of the available diffusion pathways and reduction of vacancies through REE+Y substitution. Therefore, the REE+Y content can be used as proxy for the Tc, except for fluorites with extreme compositions. - The anion concentrations and the accumulated radioactive damage density have only minor or neglectable effect on the He diffusion in fluorite. We selected the Erzgebirge as test area and we compared the well established low-T geochronometers with the age constraints obtained by the FHe method: We performed ZHe, AHe and AFT analyses on the basement rocks of the Erzgebirge, especially in the surroundings of fluorite-bearing ore deposits. According to thermal modelling the Erzgebirge is dissected by two roughly NE-SW aligned fault zones leading to three structural blocks that experienced different post-Variscan thermal histories. - Basement rocks experienced cooling to near-surface temperatures shortly after the Variscan orogeny and were then buried by Permian to Jurassic sediments. Thermal modelling suggests that the pre-Cretaceous burial was deeper in the southern to central parts of the Erzgebirge compared to its northern part. - Cretaceous hydrothermal ore-forming fluids generated local thermal anomalies and have reset all of the three thermochronometers applied. The thermal activity terminated before latest Cretaceous. - The FHe results match well to the apatite and zircon-based time constraints of the thermal history of Erzgebirge. The fluorite He ages and the diffusion parameters allowed for modelling of the thermal history. The modelled cooling paths again match well to the paths, which resulted from the AFT, AHE and ZHe input data. There are two fluorite generations in the Horni Krupka deposit with different compositions and different closure temperatures: The high- and low-closure temperature fluorites yielded Permian and Cretaceous FHe ages, respectively. These ages are in good agreement with the two characteristic periods of thermal evolution of the region, namely post-Variscan cooling and Late Cretaceous exhumation and termination of hydrothermal activity. - The dependence of the He diffusivity on the Ca -> REE+Y substitution as predicted from the laboratory diffusion experiments is thus well confirmed by the Horni Krupka case study.
Publications
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(2012) Fluorite (U-Th)/He thermochronology. The 22nd V.M. Goldschmidt Conference. Montreal, Canada. 24.-29.6.2012
Wolff, R., Dunkl, I., von Eynatten, H.
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(2012) Rare Earth Elements in Fluorite. GeoResources for the 21th century, Hannover, Germany. SDGG. 1.-03.10.2012
Wolff, R., Dunkl, I., von Eynatten, H.
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(2013) Superposition of burial and hydrothermal events: Post Variscan thermal evolution of the Erzgebirge, Germany. Geofluids, Tübingen, Germany. 15.-19.9.2013
Wolff, R., Dunkl, I., Lange, J.-M., Tonk, Ch., Voigt, T., von Eynatten, H.
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(2014) Fluorite (U-Th)/He thermochronology: what do we really date? Thermo2014, 14th International Conference on Thermochronology, Chamonix, 8.-14.9.2014
Wolff, R., Dunkl, I., von Eynatten, H.
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(2015) Variable Helium Diffusion in Fluorite. The 25nd V.M. Goldschmidt Conference. Prague, Czech Republic. 16.-21.8.2015
Wolff, R., Dunkl, I., von Eynatten, H.
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(2015). Superposition of burial and hydrothermal events: post-Variscan thermal evolution of the Erzgebirge, Germany. Terra Nova, 27, 292-299
Wolff, R., Dunkl, I., Lange, J.-M., Tonk, C., Voigt, T. von Eynatten, H.
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(2015). The age of the latest hydrothermal overprint of tin and polymetallic deposits in the Erzgebirge, Germany: constraints from fluorite (U- Th-Sm)/He thermochronology. Economic Geology, 110, 2025-2040
Wolff, R., Dunkl, I., Kempe, U., von Eynatten, H.
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(2016). Variable helium diffusion in fluorite. Geochimica et Cosmochimica Acta, 188, 21-34
Wolff, R., Dunkl, I., Kempe, U., Stockli, D., Wiedenbeck, M., von Eynatten, H.
