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Fe concentration in palaeosols and in clayey marine sediments: two case studies in the Variscan basement of Sardinia (Italy)

Published online by Cambridge University Press:  09 July 2018

P. Mameli ,
G. Mongelli ,
G. Oggiano  and
R. Sinisi
P. Mameli*
Affiliation:
Istituto di Scienze Geologico-Mineralogiche, Università degli Studi di Sassari, Corso Angioj 10, I-07100 Sassari, Italy
G. Mongelli
Affiliation:
Dipartimento di Scienze Geologiche, Università degli Studi della Basilicata Campus di Macchia Romana, I-85100 Potenza, Italy
G. Oggiano
Affiliation:
Istituto di Scienze Geologico-Mineralogiche, Università degli Studi di Sassari, Corso Angioj 10, I-07100 Sassari, Italy
R. Sinisi
Affiliation:
Dipartimento di Scienze Geologiche, Università degli Studi della Basilicata Campus di Macchia Romana, I-85100 Potenza, Italy
*
*E-mail: [email protected]
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Abstract

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.

Keywords

ironstoneschamositegoethitegeochemistrySardinia
Type
Research Article
Information
Clay Minerals , Volume 43 , Issue 4 , December 2008 , pp. 531 - 547
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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