Comparative EPMA and μ-XRF methods for mapping micro-scale distribution of iodine in biocarbonates of the Callovian–Oxfordian clayey formation at Bure, Eastern part of the Paris Basin

A pluridisciplinary approach was used to define iodine immobilization mechanisms by biocarbonates in a natural marine carbonate-bearing clayey formation. For this purpose, different techniques of observation (optical microscope, scanning electron microscope (SEM), cathodoluminescence (CL)) and of analyses (infrared spectrometry (IR), electron microprobe (EPMA), spatially resolved synchrotron-based X-ray fluorescence (μ-XRF) and X-ray diffraction (μ-XRD)) were performed on two entire and centimeter-sized carbonate shells of the Callovian–Oxfordian (160 Ma) clayey formation from the ANDRA (French Radioactive Waste Management Agency) Underground Research Laboratory (Meuse/Haute Marne, France), in the Eastern part of the Paris Basin. Combined (SEM, CL, IR and μ-XRD) data indicates that the biostructure of the Rhynchonella shell is relatively well-preserved but bioaragonite slowly transforms into calcite, whereas the bivalve shell is entirely recrystallized into diagenetic calcite and celestite. EPMA and μ-XRF data show bioaccumulation of iodine in carbonate shells, confirming previous work on present-day mollusks. EPMA analyses give evidence of iodine content up to 1200 ppm in the preserved Rhynchonella shell and up to 2000 ppm in recrystallized bivalve shell. μ-XRF elemental mapping shows that iodine is more homogeneously distributed in bio-calcite of the Rhynchonella shell than in recrystallized calcite of the bivalve shell, suggesting a loss of iodine during re-crystallization processes, but not a total exclusion of iodine from the carbonate structure. Combined EPMA data and μXRF elemental maps do not give evidence of any correlation between the iodine location and the distribution of other elements.