Abundances and chemical fractionation of Al, Fe, Mn, Zn, Pb, Cu and Tl in Cretaceous-Palaeocene limestones from Gubbio (Central Italy)

Abstract

Partitioning of Al, Fe, Mn, Zn, Pb, Cu and Tl in pelagic carbonate rocks (29 samples) has been determined by sequential chemical leaching, performed with 1 M NH₄OAc (easily extractable fraction), 0.8 M HOAc (carbonate component), 1 M NH₂OH HCl and dilute HOAc (hydrous oxides) and HF-HClO₄ (residual phases). The analytical results show great homogeneity among the samples, even though they cover a 20m.y. time-span. Mn, Zn, Pb and Cu are associated with the carbonate fraction in high percentages. Hydrous oxides, very likely because they are almost Mn-free, do not carry any significant amount of metals. Fe exhibits high percentage in residual fraction, probably because well-crystallized Fe oxides are not dissolved by NH₂OH·HCl/HOAc treatment. Tl is totally contained in residual phases, and as it shows a good, positive correlation with K, it is inferred to be carried by K-bearing silicates. The distribution pattern of elements is interpreted as the result of the redox conditions to which the sediments were submitted during the sedimentation as well as early diagenesis.