Chemical processes affecting the mobility of major, minor and trace elements during weathering of granitic rocks

Abstract

The behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some Portuguese granitic rocks. Element mobilities are calculated from geochemical data normalized with respect to Ti in the fresh parent rock. Chemical elements are divided into two groups, immobile and mobile, on the basis of their geochemical distribution during weathering. Elements that are immobile during weathering are Zr, Hf, Fe, Al, Th, Nb, Sc and the REE. Very mobile are Ca, Na, P, K, Sr, Ba, Rb, Mg and Si. Mobile elements are derived mainly from leachable minerals such as feldspars, micas and apatites, whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals. The geochemical behaviour of Mn, Cr, V, Fe and Ce is very dependent on redox conditions. Redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite. The REE are mobilized or fractionated during late stages of weathering, but not during moderate stages of weathering. This fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing REE.