Trace element variation in size-fractionated African desert dusts

Desert particulate samples from locations in the Sahara–Sahel dust corridor (Western Sahara, Algeria, Chad and Niger) were segregated into size fractions ranging from <1.5 to >20 μm and analysed using inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). The size-fractionated samples contain higher concentrations of Al, Na, Mg, Fe and most trace elements in the finer, phyllosilicate-rich materials. A notable exception is provided by Zr and Hf, which concentrate in the coarser fractions due to a higher abundance of coarse detrital zircon. Equally explicable by natural processes are enrichments (relative to the average upper continental crust) in incompatible trace elements with higher ionic potential (Nb, Ce, La), these being more resistant to chemical weathering. In contrast, samples show a relative loss of large ion lithophile elements (Rb, Cs and Ba), and the more calcareous samples show relative enrichment in Sr and depletion in Nb. However, not all the geochemical variations exhibited by these samples are easily explicable by invoking “natural” geochemical processes: anomalous concentrations of several transition metals (Zn, Pb, Cd) can be attributed to anthropogenic pollution, and enrichments in Cr, Ni and Mo are due to abrasive interaction between the siliceous dust sample and the fractionation equipment.