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| Titel |
Dating of polyhalite and langbeinite: preliminary results from German Zechstein |
| VerfasserIn |
Franz Neubauer, Anja Schorn, Christoph Leitner, Johann Genser |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079524
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| Zusammenfassung |
| Evaporite mélanges often form decollément surfaces of major extensional and contractional
allochthons because of the very low shear resistance of halite. Due to the deposition of
evaporites during an early stage of passive continental margin formation, evaporites are
commonly overlain by thick successions of carbonates and/or siliciclastic rocks deposited
during the main thermal subsidence stage of the passive margin formation. The most common
cases of evaporite mélanges are such (1) at passive continental margins, where they are
deformed during gravity-driven extension, commonly raft tectonics, in an extensional
geodynamic setting, (2) in external foreland fold-thrust belts within a convergent geodynamic
setting, and (3) in salt diapirs. In all these cases, halite is strongly deformed by late-stage
deformation and only sulphate lenses composed of anhydrite and gypsum preserve early
deformational stages. Dating of K-sulphates may allow the recognition of early stages of
deformation although this method is poorly applied (Renne et al., 2001). Knowledge of the
limitations of K-sulphate chronometers of langbeinite and polyhalite may allow,
therefore, dating of full history of evaporite mélanges (for polyhalite, see Leitner et al.,
2012).
Polyhalite has the chemical formula [K2Ca2Mg(SO4)4-
2 H2O] and commonly occurs in
sedimentary evaporite successions. The mineral can be synthesised under laboratory
conditions by a reaction of gypsum with appropriate solutions in the ternary system
K2SO4–MgSO4–H2O at temperatures above 70 Ë C (Freyer and Voigt, 2003). At lower
temperatures, polyhalite crystallisation slows down (Wollmann, 2010). In nature,
polyhalite, which is stable between ~ room temperature (0–25 Ë C) and 255–343 Ë C
(Wollmann et al., 2008) or 285 Ë C (Fischer et al., 1996), most commonly forms
early-diagenetically or secondarily (Warren, 2006 and Leitner et al., 2012 and references
therein).
The secondary mineral langbeinite [K2Mg2(SO4)3],whose lower formation temperatures
are given between 57.1 Ë C or more commonly 83 Ë C (e.g. Neitzel, 1992 and
references therein), is mined as potash ore at the German Zechstein deposits. Neitzel
(1992) summarized two main types of langbeinite formation (1) from kainite and
halite (due to thermal metamorphism) and (2) from sylvinitic Hartsalz (= mixture of
sylvite, kieserite and halite) due to solution metamorphism. The mineral might also
form by decomposition of polyhalite to langbeinite and anhydrite during prograde
metamorphism.
In the following, we discuss the first successful results of polyhalite and langbeinite
dating in Zechstein salts of Germany (Morsleben, Neuhof). Extremely fine-grained
(grain sizes < 10 x 10 μm) recrystallised polyhalite from Morsleben gave an age
of ca. 28.68 ± 0.11 Ma, which may represent the age of crystal growth from a
brine. Dating of deformed langbeinite from a mylonite zone from Neuhof gave a
slightly scattered age pattern at ca. 150 Ma, implying a major step of ductile flow of
K-bearing evaporites and crystallization of langbeinite. From analytical point of view,
langbeinite is very stable and allows diffusion experiments over a wide range of
energies.
References
Fischer, S., Voigt, W., Köhnke, K., 1996. The thermal decomposition of polyhalite K2SO4
. MgSO4. 2 CaSO4. 2 H2O. Crystal Research and Technology, 31, 87-92.
Freyer D., Voigt W., 2003. Crystallization and phase stability of CaSO4 and CaSO4-based
salts. Monatshefte für Chemie, 134, 693–719.
Leitner, C., Neubauer, F., Marschallinger, R., Genser, J., Bernroider, M., 2012. Origin of
deformed halite hopper crystals, pseudomorphic anhydrite cubes and polyhalite in
Alpine evaporites (Austria, Germany). International Journal of Earth Sciences, DOI
10.1007/s00531-012-0836-6.
Neitzel, U., 1992. 100 Jahre Langbeinit. Kali und Steinsalz, 11/1, 7–13.
Renne, P. R., Sharp, W. D. Montañez, I. P., Becker, R. A., Zierenberg, R. A., 2001.
40Ar/39Ar dating of Late Permian evaporites, southeastern New Mexico, USA. Earth and
Planetary Science Letters, 193, 539–547.
Warren, J. K., 2006. Evaporites: Sediments, Resources and Hydrocarbons. Springer,
Berlin Heidelberg, 1035 p.
Wollmann, G., 2010. Crystallization Fields of Polyhalite and its Heavy Metal Analogues.
Ph.D. thesis, University Bergakademie Freiberg.
Wollmann, G., Freyer, D., Voigt, W., 2008. Polyhalite and its analogous triple salts.
Monatshefte für Chemie, 139, 739–745. |
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