Trace element mapping in vein calcite with synchrotron XFM: implications for U-Pb geochronology

U-Pb geochronology via Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) is a fast and reliable method for in-situ dating of calcite that is used across disciplines in earth science. However, the heterogeneous distribution of U (and Pb) in individual calcite crystals represents a yet unmitigated challenge and identifying zones of sufficiently high U concentrations that can provide precise constraints on timing of calcite precipitation is an inefficient “hit or miss” process. Moreover, it is challenging to confirm that targeted domains of a calcite crystal retain their pristine geochemical signature, given the range of post-crystallization dissolution-reprecipitation and solid diffusion processes that can affect this mineral. There is thus an urgent need to understand the spatial and temporal mechanisms of U incorporation and mobilization in calcite to ultimately improve this key geochronological tool. To determine where specific trace elements are located within calcite crystals, investigate how they are incorporated during crystal growth and how they are affected by post-crystallization fluid-assisted deformation processes, we applied Synchrotron X-Ray Fluorescence Microscopy (XFM) with emphasis on U mapping, Electron Backscatter Diffraction (EBSD), and LA-ICP-MS to a set of calcite veins. Samples were collected from drillcores through the Middle and Upper Jurassic carbonates and marls (max. 85°C) in the Neogene Molasse Basin in central northern Switzerland. By combining high-resolution trace element maps with information on the crystal lattice structure of calcite we show two main textural types of trace element distributions within syntaxial calcite veins: 1) oscillatory crystal growth zonations that reflect preferential incorporation of trace elements into structurally different growth steps and faces of growing calcite crystals during growth and, 2) complete overprint of the initial growth zonation upon potential secondary fluid infiltration and trace element replacement. The anti-correlation between Fe, Mn and Sr, U demonstrates the role of kinetic factors during trace element partitioning between fluid and calcite, pointing to the inhibition of Fe incorporation at higher growth rates. Where the Sr uptake during calcite growth is generally enhanced with growth rate. The results of this project give valuable insights in the complexity of fluid overprint during multi-staged deformation cycles in the modification of trace elements in calcite, with clear implications for the applicability and reliability of U-Pb geochronometer in calcite.