⁴⁰Ar/³⁹Ar ages of crystallization and recrystallization of rock-forming polyhalite in Alpine rocksalt deposits

Polyhalite rocks of the evaporitic Haselgebirge Formation are emplaced within a rocksalt–mudrock tectonite. The rheologically weak series served as a major detachment level during nappe stacking of the Northern Calcareous Alps (Eastern Alps). To test the mineral polyhalite [K₂Ca₂Mg(SO₄)₄·2H₂O] as a useful geochronometer for various diagenetic and deformation fabric types, ⁴⁰Ar/³⁹Ar age dating was combined with microstructural analysis. Vein infills, polyhalite intergrown with anhydrite and polyhalite within mudrock, crystallized in several stages between c. 235 and 210 Ma. Mylonites of fine-grained polyhalite rock indicate subsequent stages of tectonothermal overprint between c. 155 and 105 Ma, which is roughly consistent with previously measured feldspar and muscovite ⁴⁰Ar/³⁹Ar data from the region. Illite crystallinity points to temperatures of c. 200 °C. The peak temperature of overprint was at c. 180 °C in the Berchtesgaden mine (vitrinite reflectance, fluid inclusions) and >240 °C in the Altaussee mine (fluid inclusions). These temperatures are below the value of 255 °C, where polyhalite starts to dehydrate. Disturbed age spectra patterns result from multiphase polyhalite growth; however, single phases and completely recrystallized fabrics yield good results. As in the Alpine test case, polyhalite may characteristically serve as a geochronometer for diagenetic and very-low-grade metamorphic processes.