Geochemical fingerprints in topsoils of the volcanic Brava Island, Cape Verde

Abstract

Topsoils developed on different lithologies of all the geological units of the volcanic Brava Island (Cape Verde archipelago) were studied by neutron activation analysis and Mössbauer spectroscopy. Sampling was performed according to the recommendations of the IGCP 259 (“International Geochemical Mapping”). Significant chemical contents variations were found even within the same volcano-stratigraphic unit, inherited from the parent rock composition under the semi-arid climate of the island. The chemical heterogeneity of topsoils within the same geological unit, particularly in the Upper Unit, was evidenced by a multivariate statistical analysis. Some differences found can be related with the geographical location/underlying basement. High contents of Mn, Co, Ga, Ba, La, Ce, Nd, Sm, Eu, Tb, Ta, W, Th and U were observed in soils related to carbonatites and phonolites. REE and W can be used to trace outcrops of extrusive carbonatites. In general iron is strongly oxidized occurring in the silicates structure as well as in hematite, oxidized magnetite and maghemite. The global iron oxidation degree, the fraction of Fe3+ in silicates and the fraction of nanosized oxides, which are higher in topsoils of the older units, may be an indicator of weathering degree. As, Br and Sb contents were also found to increase with weathering. Topsoils developed on extrusive carbonatites are clearly distinguished due to the absence of Fe2+ in the silicate phases and the remarkable predominance of low oxidized magnetite.