REE distribution in the clay fraction of pelites from the southern Apennines, Italy

In order to evaluate whether clay minerals or accessory phases control the REE pattern of the <2-μm fraction of pelitic sediments, a set of samples from a borehole in the southern Apennines, Italy, was analysed for mineralogy and chemical composition (major elements, Rb, Sr, Y, Zr, Nb and REE). SEM analyses showed the presence of accessory phases such as Ti-oxides, apatite, zircon and pyrite. XRD procedures revealed that the <2-μm fraction essentially consists of illite, smectite, dickite, kaolinite and chlorite. Illite and smectite probably derived by leaching from a micaceous “biotite-like” mineral while dickite took origin from post-depositional, interstitial fluids. Only illite is correlated with REE, while a smectite-rich sample is characterized by the lowest content in REE. Strong correlations exist between REE and TiO2, Nb, Zr and a less significant one with Sr; no correlations exist between these elements and Y. R-mode factor analysis indicated that three factors account for 87% of the total variance. The first factor (Var = 43.2%) seems to express mainly the tendency of some resistate minerals to concentrate REE in the fine fraction. The second factor (Var=30.0%) may be related to the effect of circulating fluids responsible for dickite genesis. The third factor may be linked to the weathering of micaceous phases. The REE do not have a significant weight in the second and third factors. Resistate minerals appear dominant in controlling, at least, the LREE and Nb. It is proposed that in the studied samples, illite likely conveyed REE via the included minerals.