Fluorite (U-Th-Sm)/He thermochronology

by Reinhard Wolff
Cumulative thesis
Date of Examination:2015-09-09
Date of issue:2016-07-01
Advisor:Dr. István Dunkl
Referee:Prof. Dr. Hilmar von Eynatten
Referee:Prof. Dr. Jonas Kley
Referee:Prof. Dr. Gerhard Wörner
Referee:Prof. Dr. Gregor Markl
Referee:Prof. Dr. Andreas Pack
Persistent Address: http://dx.doi.org/10.53846/goediss-5705

 

 

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Abstract

English

The aim of my thesis is to establish an emerging and novel method in low-temperature thermochronology based upon the mineral fluorite. The new and until now rarely applied method, fluorite (U-Th-Sm)/He thermochronology (FHe), is validated in two ways: by (A) analytically estimating the helium diffusion which controls the closure temperature (Tc) by step heating experiments and (B) comparing it to well-established thermochronometers in an empirical case study. Regarding the first, the diffusion experiments indicate a thermally activated process with the physical crystal as the diffusion domain. Moreover, the diffusion data allows one to draw the conclusion that the closure temperature is variable. Hereby, the calculated closure temperature of the fluorite thermochronometer varies between 46 ± 14 °C and 169 ± 9 °C, considering a 125 µm fragment size. (log D0/a2 = 0.30 ± 0.27 to 7.27 ± 0.46 s-1 and Ea = 96 ± 3.5 to 182 ± 3.8 kJ/mol). In this thesis I establish that minor substitutions of calcium by rare earth elements and yttrium (REE+Y) and related charge compensation by sodium, fluorine, oxygen and/or vacancies in the fluorite crystal lattice have a significant impact on the diffusivity of helium in the mineral. With increasing REE+Y concentrations F vacancies are reduced and key diffusion pathways are narrowed. Consequently, a higher closure temperature is to be expected. Therefore, the REE+Y content can be used for estimating the Tc. An empirical case study in the Erzgebirge, Germany confirms this variability: The complex post Variscan thermal history of the Erzgebirge is established with apatite and zircon (U-Th)/He and apatite fission track thermochronology. It is the result of Mesozoic sedimentary burial, exhumation in Cenomanian times and superimposed Cretaceous hydrothermal activity. The present-day surface of the Erzgebirge was exhumed to a near-surface position after the Variscan orogeny. Thermal modelling reveals a Permo-Mesozoic burial up to temperatures of 80 to 100 °C while the sedimentary cover thins out towards the north resulting in maximum burial temperatures less than 40 °C. This thermal pattern was modified locally by Cretaceous hydrothermal activity that resets the zircon (U-Th)/He thermochronometer along ore veins. In line with the empirical case study, 233 fluorite aliquots taken from 38 samples at 7 localities from two mineralisation types (Sn-W mineralisation of Late Carboniferous to Early Permian age and a polymetallic vein mineralisation of probably Mesozoic age; they can be distinguished by La-Er-Gd using linear discriminant analysis) have been dated. Six deposits yield Cretaceous FHe ages (112 ± 10 to 79 ± 10 Ma) independent of their paragenesis, while samples from the Sadisdorf Sn-W deposit yields 234 ± 16 Ma. The younger ages are interpreted as cooling ages indicating the time when the last thermal overprint, including possible hydrothermal activity, in the Erzgebirge ceased. The oldest, Triassic fluorite ages at Sadisdorf still carry a memory of the late Variscan mineralisation because Mesozoic thermal overprint performed partial reset only. The Triassic FHe ages are thus considered as mixed ages. Thermal modelling based on FHe ages and He diffusion parameters in fluorite yields thermal histories comparable to the results from the well-established apatite- and zircon-based thermal modelling. Empirical observation of two fluorite samples from the same deposit (Horni Krupka, Czech Republic) with ca. 170 °C and ca. 43 °C Tc yield highly different (U-Th-Sm)/He ages of 290 ± 10 Ma and 79 ± 10 Ma, respectively. Accordingly, the fluorite sample with the high Tc could have quantitatively retained helium since the formation of the fluorite-bearing ores in the Permian, despite subsequent Mesozoic burial and associated regional hydrothermal heating. In contrast, the fluorite with the low Tc yields a Late Cretaceous age close to the apatite fission track and apatite (U-Th)/He ages from the same locality.
Keywords: fluorite; low-temperature thermochronology; (U-Th)/He thermochronology; fission track; thermal modelling; diffusion; noble gas diffusion; apatite (U-Th)/He; zircon (U-Th)/He; Erzgebirge; ore deposits; rare earth elements
 
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