MOBILITY OF MAJOR, MINOR AND SOME REDOX-SENSITIVE TRACE-ELEMENTS AND RARE-EARTH ELEMENTS DURING WEATHERING OF 4 GRANITOIDS IN CENTRAL PORTUGAL

Element mobility was studied in four different granitoids in central Portugal. Samples of fresh and weathered rock parts were collected from each type of granitoid, An increase in the degree of weathering is accompanied by reductions in Na2O, CaO and SiO2 (plagioclase), P2O5 (apatite), and in most cases also by reduction in K2O (biotite, potash feldspar). Losses as well as gains are observed for MgO. Relative changes in the Al2O3 contents are attributed to physical transport or accumulation of fine weathering products. Mass-balance calculations indicate that kaolinite and in some cases also gibbsite are the dominant weathering products; this is in agreement with X-ray diffraction analyses. Manganese is found to decrease but not Fe. There is a high correlation between V and Fe; this is preserved during weathering. There were both increases and decreases in the trace elements Cr, As and Sb; U had become slightly enriched in the weathered parts. No systematic trends in REE behaviour were observed, Differences in the behaviour of these elements and in their ratios can be explained by a combination of factors such as differential dissolution of the host minerals, redox conditions and redox kinetics, complexation by organic and inorganic ligands, and sorption. The interpretation of the results is complicated by apparent inhomogeneities in the granites on the mesoscale of an outcrop.