Dating of polyhalite: a difficult (40)Ar/(39)Ar dating tool of diagenetic to very low-grade metamorphic processes

Halite already deforms at surface temperatures. A valuable universal dating tool to constrain the timing of sedimentary, diagenetic, or deformational structures is still missing. The evaporite mineral polyhalite can be dated by the (40)Ar/(39)Ar method. On the example of the extremely deformed halite deposits of the Eastern Alps, polyhalite was tested to date early diagenetic stages of the deposits. The sedimentological investigation of the present study indicates that some of the macrostructures of polyhalite had a syn-depositional origin during the late Permian. It is supposed that polyhalite originated during reflux of brines. All samples selected for age dating represent characteristic microfabric types of euhedral to subhedral polyhalite crystals. Intact macro- and non-recrystallized looking microstructures of polyhalite can be expected to give plateau ages. However, nearly all measurements produced overdispersed data that do not define an age. The oldest age steps thus represent only minimum ages. A closer look revealed grain boundary migration, subgrain rotation recrystallization, twinning, and fluid-supported grain size increase. These recovery processes obscured the original ages and/or reflect the origin of new polyhalite in place of the original individuals. Based on these microstructures, the age data are supposed to reflect the circulation of aqueous fluids. Just extremely careful separation of individual crystals or in situ age dating under the microscope will be successful in dating polyhalite. Nevertheless, polyhalite can potentially serve to date deformational events of halite deposits due to its easy recrystallization property. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00531-022-02219-9.