Polyhalite

The project intends to study the potential use of polyhalite, a K-Ca-Mg-sulfate, for dating low-temperature processes and tectonic deformation of tectonic evaporite mélanges by the 40Ar/ 39Ar method. The work will likely not only allow to unravel the tectonic history of evaporite mélanges, which are common in many thin-skinned thrust-fold belts worldwide but also - in the future - could possibly contribute to the exploration of the early geological history of planet Mars, where polyhalite has been detected in widespread evaporites. Polyhalite as a potential new low-temperature geochronometer will provide insight into geological processes ocurring in the range between surface temperature and ca. 300C. We intend to examine the behavior of the argon isotopic system in polyhalite-bearing rocks using a combined approach: It will include the study of microfabrics and age-dating of polyhalite with the 40Ar/ 39Ar system. No such strategy has so far been pursued and we presently know very little about these topics. Evaporite mélanges often preserve a wide variety of otherwise unusual rock fabrics, formed during a number of geological processes ranging from sedimentary deposition to diagenetic, thermally controlled mineral growth and final emplacement of evaporites as diapirs or along thrust surfaces. In many cases, the tectonic evaporite mélange comprises various blocks of country rocks, which were incorporated during the uprise of diapirs and/or final emplacement during orogenic shortening. Consequently, the entire history from deposition to the final emplacement could be revealed in such evaporite mélanges as long as the temperature remains below ca. 250- 350C, the upper stability limit of polyhalite. The proposed work will include (1) 40Ar/ 39Ar dating of polyhalite formed during distinct geological processes, (2) characterization of microfabrics of polyhalite-bearing rocks such as rocksalt and anhydrite, (3) 40Ar/ 39Ar biotite as well as zircon and apatite fission track analysis of magmatic and siliciclastic blocks in order to cross-check the results from step (1). The (proposed) study will focus on the evaporitic Haselgebirge mélange of Permian to Early Triassic depositional age in the Eastern Alps. This mélange is suspected to represent an important suture zone at a high structural level of the Eastern Alps, which potentially correlates with the Meliata suture of the Western Carpathians. Results obtained here will be cross-checked by a companion study on polyhalite samples collected in Upper Permian Zechstein evaporites of Northern Germany. There, polyhalite occurs in a number of different microfabrics, some of them representing the result of secondary water inflow and some recrystallized during orogenic shortening. The final results will have influence on the assessment of the long-time stability of diapirs, which are used as repositories for nuclear waste (e.g. Morsleben), the future possible use of polyhalite as a mineral fertilizer and even for planetary geology as polyhalite will likely allow date early sedimentation on Mars.

The project was dealing with sulphatic evaporite mélanges, its structures and structural development. Such evaporite successions are widespread in many mountain belts (like Alps) and in continental platform regions (e.g. northern Germany). The project resulted in progress in three directions: We were able to demonstrate the valuable use of two K-bearing sulphates (polyhalite and langbeinite) for dating major overprint events in buried evaporite successions. Polyhalite is stable between ca. near-surface temperatures and ca. 300C, whereas langbeinite likely forms by decomposition of polyhalite at higher temperatures. Polyhalite is quite common, whereas langbeinite is identified in a few places only. The results from the Permian Zechstein in Germany demonstrate that langbeinite was formed during ductile shear events between 150 and 136 Ma before present, in accordance with other geological evidence. In Alps, langbeinite is rare but allowed demonstrate rapid burial of sulphates and post-depositional formation of langbeinite at ca. 241 Ma before present. Polyhalite dating gave evidence for a number of postdepositional events, which resulted in formation of a wide variety of microfabrics. These data allow mainly demonstrate fluid-rock interaction during extensional events (ca. 235 230 Ma and at ca. 210 Ma before present), main shearing during extension (at ca. 200 and 160 Ma before present) and compression occurred between 140 and 135 Ma before present. The rocks were finally overprinted by a thermal event ca. 45 and 35 Ma before present. These data allow now a much more detailed insight into the tectonic evolution of Alpine evaporate successions as it was possible up to now. Evaporite mélanges are often formed by overprint of contractional structures formed within a fold-thrust belt over structures of raft tectonics during passive continental margin setting. We developed distinct criteria how to distinguish between structures of these two contrary tectonic settings. We also developed new methods and textural and microfabric characteristics of wide range of materials typical for sulphates and halite. These include new observations of a new halite paleopiezometer, which demonstrate the rapid plastic deformation of halite in Alpine salt deposits, which is continuing until recent times. In summary, the final results allow recognition of early structures in evaporite successions in mountain belts and platform successions, the assessment of the long-time stability of diapirs, which are used as repositories for nuclear wastes, the future potential use of polyhalite as a mineral fertilizer and, in future, may even allow date sedimentary sulphates on planet Mars.