Paleoweathering of intrusive carbonatites (4.5 Ma) from Fogo Island, Cape Verde – Mineralogy and geochemistry

Abstract

Intrusive carbonatites from a fluvial valley near Monte Almada (Fogo Island, Cape Verde), corresponding to the oldest unit of the island (≈4.5 Ma) occur under alkaline lavas and tuffs of more recent age in two slopes, with a very steep inclination. Its exposure with the water line fitting led to the weathering of the outcrops, and the materials are currently retained in small terraces with shrubs and trees. The motivation that drives this work was the absence of paleoweathering materials studies of these carbonatites in Fogo Island. Mineralogical and chemical analyses of paleoweathered carbonatites samples with different weathering degrees were performed by means of X-ray diffraction (XRD) and instrumental neutron activation analysis (INAA). The results obtained from fresh rock to near paleosurface samples showed that chemical contents could vary significantly in the fresh rock, particularly the trace elements, including the rare earth elements (REE), as well as in the size fractions. Besides calcite, pyroxenes and phyllosilicates, traces of anatase and K-feldspars also occur in the <2 mm fraction; dolomite was detected in one sample. Smectite, illite and kaolinite were found in the <2 µm fraction. Carbonatites weathered products are easily distinguished from other volcanic soils of Fogo Island due to higher REE contents, particularly the light and middle ones, and to higher K, Rb, Cs, W, and Th contents. Higher concentrations of Zn and Cs are observed in the silt-clay and clay-size fractions where phyllosilicates dominate, particularly smectite and illite.