Iron concentrations in palaeosoils and in clayey marine sediments: two study cases from the variscan basement of Sardinia (Italy).

Within the Variscan basement of Sardinia (Italy), two main Fe concentrations occur in the low-grade metamorphic tectonic units: (1) an uppermost-Ordovician oolitic ironstone of shallow anoxic water environment; and (2) a concentration of Fe oxyhydroxides lying on a palaeosurface. Two sets of samples were picked from the marine ironstone and from the continental Fe concentration following stratigraphic criteria. Chemical analysis, X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis and thin-section studies were performed on 34 samples. Marine ironstones formed under a highly reducing, anoxic, non-sulphidic methanic environment, and their Fe phases are chamosite, siderite and magnetite. Detrital chlorite and illite, produced during physical weathering, were chamosite precursors. Using the V/Cr proxy, an emergence stage that caused a transition to an oxic environment is documented. In contrast, continental ironstones formed under oxic conditions and the dominant Fe phase is goethite, which can adsorb Zn2+ and U6+. Unexpected negative Ce anomalies occur in this set of samples, suggesting that the oxyhydroxides originated from Ce-depleted solutions. Although the ironstones of Sardinia formed in different environments (marine vs. continental) and under contrasting climatic conditions (sub-glacial vs. tropical) they share similar geochemical features. These dramatic palaeoenvironmental differences did not result in large differences between the geochemistry of the chemical sediments.