The conditions controlling the formation of sedimentary dolomite are still poorly understood despite decades of research. Reconstructing formation temperatures and δ18O of fluids from which dolomite has precipitated is fundamental to constrain dolomitization models. Carbonate clumped isotopes are a very reliable technique to acquire such information if the original composition at the time of precipitation is preserved. Sedimentary dolomite first mostly forms as a poorly-ordered metastable phase (protodolomite) and subsequently transform to the more stable ordered phase. Due to this conversion its important to determine if the original clumped isotope composition of the disordered phase is preserved during diagenetic conversion to ordered dolomite, and how resistant clumped isotope signatures are against bond reordering at elevated temperatures during burial diagenesis. Here, we present a series of heating experiments at temperatures between 360 and 480 °C with durations between 0.125 and 426 hours. We uses fine-grained sedimentary dolomites to test the influence of grains size, and cation ordering on bond reordering kinetics. We analyzed a lacustrine dolomite with poor cation ordering and well ordered a replacement dolomite, both being almost stoichiometric. The poorly ordered dolomite shows a very rapid alteration of its bulk isotope composition and higher susceptibility to solid state bond reordering, whereas the well-ordered dolomite behaves like a previously studied coarse-grained hydrothermal dolomite. We derive dolomite-specific reordering kinetic parameters for ordered dolomitea and show that ∆47 reordering in dolomite is material specific. Our results call for further temperature-time series experiments to constrain dolomite ∆47 reordering over geologic timescales.

During its late-stage evolution, the European Alpine orogen witnessed a northwest-directed propagation of its deformation front along an evaporitic basal décollement into the foreland. This resulted in the decoupling of the northern Alpine Molasse Basin from its basement and the formation of the Jura fold-and-thrust belt. Here, we present the first absolute age and temperature constraints on deformation along this basal décollement using combined carbonate U-Pb LA-ICP-MS dating and clumped isotope thermometry. We analyzed calcite veins associated with a thrust fault branching off from the basal décollement in the distal Molasse Basin and slickenfibers from thrusts and strike-slip faults in the eastern Jura Mountains. Our U-Pb data provide evidence for tectonic activity related to Alpine contraction in this region between ~14.5 Ma and ~4.5 Ma ago. Accordingly, the propagation of Alpine deformation into the distal foreland along the basal décollement occurred earlier than commonly inferred by biostratigraphy, at Middle Miocene (Langhian) times at the latest. Younger deformation ages between ~11.5 and ~4.5 Ma correspond very well in time with shortening in the Subalpine Molasse and the Central Alps proving simultaneous tectonic activity along both thrust fronts; e.g. the Jura Mountains and the Subalpine Molasse. Clumped isotopes reveal vein calcite precipitation at temperatures between 53 and 104 °C from fluids with oxygen isotope compositions between -6.2 and +9.5 ‰. Our data show that the burial conditions in the studied area remained constant between ~14.5 Ma and ~4.5 Ma indicating that the previously reported large-scale foreland erosion initiated after ~4.5 Ma.