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

Mameli, Paola and Mongelli, Giovanni and Oggiano, Giacomo and Sinisi, Rosa (2008) Fe concentration in palaeosols and in clayey marine sediments: two case studies in the Variscan basement of Sardinia (Italy). Clay Minerals, Vol. 43 (4), p. 531-547. eISSN 1471-8030. Article.

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DOI: 10.1180/claymin.2008.043.4.02

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.

Item Type:Article
ID Code:4644
Status:Published
Refereed:Yes
Uncontrolled Keywords:Ironstones, chamosite, goethite, geochemistry, Sardinia
Subjects:Area 04 - Scienze della terra > GEO/09 Georisorse minerarie e applicazioni mineralogico- petrografiche per l'ambiente ed i beni culturali
Area 04 - Scienze della terra > GEO/03 Geologia strutturale
Divisions:001 Università di Sassari > 01 Dipartimenti > Scienze botaniche, ecologiche e geologiche
Publisher:Mineralogical Society
eISSN:1471-8030
Deposited On:12 Oct 2010 10:27

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