A multianalytical X-ray based micro-spectroscopic approach for the analysis of carbonates in beachrock cements

Abstract

The molecular micro-characterization and the estimation of the Mg content of CaCO₃ cements are crucial to define the environmental biophysicochemical conditions in which unusual temperate latitude beachrock formations materialize. To this end, petrographic investigations and a multianalytical micro-spectroscopic approach based on X-ray techniques (Micro-Energy Dispersive X-ray Fluorescence Spectroscopy (μ-EDXRF), Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM-EDX) and X-ray Diffraction (XRD)) were at first applied on beachrocks. Such instruments provided the perfect acquaintance for the microscopic inspection of the main metastable CaCO₃ cements identified. The Cement Generation 1 (CG1) (5–10% of the total carbonates) constituted a 15–20 μm thick layer of micron-sized rhombohedral crystals of high magnesium calcite (HMC) (Ca(Mg)CO₃) (7.4–12.8% mol MgCO₃). The mineralogy and the trace elements (Si, Al, Na, P, S, Cl, K and seldom Fe) found within that cement suggest organomineralization activities encouraged by microbiological processes in the first stages of beachrock formation. This is also consistent with the microbiological degradation of the organic matter deposited in the coastal sediments derived from the outfalls of an urban sewage plant. The Cement Generation 2 (CG2) accounted for 70–85% of the carbonaceous cements, with isopachous 60–120 μm thick pure fibrous/acicular crystals of orthorhombic aragonite (CaCO₃) growing vertically from the CG1 to the pore spaces. The results advocate for a cement precipitation in marine vadose and phreatic subenvironments with an initial saturation of pore-water chemistry promoted by organomineralization activities. The followed multianalytical procedure proved to be a conquering approach for the elemental, molecular and structural micro-stratigraphic characterization of carbonate cements.