Extrusive carbonatite outcrops – A source of chemical elements imbalance in topsoils of oceanic volcanic islands

Extrusive carbonatite ash tuff (Tantum) and spatter (Monte Miranda) deposits, and topsoils (incipient and shallow) from both sites of Brava Island (Cape Verde archipelago) were studied by neutron activation analysis, X-ray diffraction and Mossbauer spectroscopy. The ash tuff deposit near Tantum, here reported for the first time, corresponds to an extrusive calciocarbonatite with high W, Th, U and REE contents and a positive Eu anomaly. Similar REE patterns and W concentrations were also found in the spatter deposit of Monte Miranda. Magnetite is the major iron containing phase in both extrusive carbonatite deposits. High W and REE concentrations associated with the presence of calcite prevail in topsoils with extrusive carbonatite influence. In the topsoil developed on extrusive carbonatite ashes, magnetite is well preserved particularly in the coarser fractions probably due to the recent age of the parent material associated with the semi-arid climate of the island; a significant chemical heterogeneity of the whole samples as well as of the different size fractions of these topsoils was found, with an enrichment of Co, Zn, Ga, As, Br, Rb, Ce, and particularly Sb and W in the clay-size fraction, which may be relevant for the study of environmental health issues in Brava Island.