In situ Rb/Sr dating of potassium salt minerals (sylvite and carnallite) by 157 nm LA-ICP-MS/MS

Abstract

Evaporite deposits are of increasing societal importance as potential repositories for high-level radioactive waste and host lithologies for the subsurface storage of natural gas and hydrogen. To conduct robust long-term risk assessments (particularly with respect to the timing of deposition and deformation), the geological history of such deposits must be reconstructed accurately. However, direct geochronometric methods applicable to evaporites remain very limited and ages may be obtained only indirectly via dating of detrital minerals. We report the first application of reaction-cell laser ablation-inductively coupled plasma-tandem mass spectrometry (LA-ICP-MS/MS), using a recently developed dual-wavelength laser system (193 nm and 157 nm), to potassium salt minerals (sylvite and carnallite) from the Morsleben site (NE Germany), which is hosted within a salt structure of Permian age. Compared with 193 nm, ablation at 157 nm is more controlled and shows less Rb–Sr elemental fractionation. Despite low (6–40 ng g⁻¹) strontium concentrations, the salt minerals contain almost exclusively radiogenic ⁸⁷Sr (∼99%), which enables the determination of precise (∼3%) single-spot Rb–Sr ages that are independent of the initial ⁸⁷Sr/⁸⁶Sr. Kernel density estimates of these single-spot ages reveal four dominant age modes at ∼4 Ma, ∼14 Ma, ∼33 Ma and ∼91 Ma. These ages are consistent with the few previously published data from Morsleben and nearby salt mines, and could be linked to regional tectonic events. We demonstrate, for the first time, the potential of in situ LA-ICP-MS/MS as a direct (and rapid) geochronological tool for evaporite deposits at a spatial resolution of ≤100 µm.