Raman Line Scan Analyses and a New Hypothesis for the Formation of Anatase‐Hosting Microbialites: Study of the Stromatolites From Saint‐Jean‐de‐la‐Rivière, Cotentin, France

Raman spectroscopy applied to rocky objects usually relies on single‐point laser excitations and two‐dimensional spectral mapping. The present article describes spectrum‐by‐spectrum analyses performed at the micrometer scale along Raman single‐line scans that are recorded on the millimeter scale. The method is applied to Lower Cambrian stromatolites from Saint‐Jean‐de‐la‐Rivière, Armorican Massif, France. Raman lines, 0.5–2.5 mm in length, drawn across stromatolitic column boundaries, show calcite as the main matrix mineral, dolomite restricted to intercolumnar spaces, high concentrations of kerogen, here specified as graphogen , accumulating at columnar margins, and an anatase–graphogen association both inside columns and intercolumnar spaces. A precise mineral zonation is determined along millimeter lines. The study of highly resolved Raman lines of spectra reveals systematic mineral associations leading to new hypotheses for the construction of the host rock. The anatase–graphogen pair is interpreted in a new model for microbialite formation that involves interactions between anatase and cells. The proposed model, involving anatase‐driven photodecomposition of microbial organic matter, can be tested on all microbialites in which anatase and graphogen co‐occur.