Fe concentration in palaeosols and in clayey marine sediments: two case studies in 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.