ABSTRACT

The concentrations of major elements and rare earth elements (REE) of the stream waters from the Rokko granite area, Japan, were analyzed to examine the relationship between the degree of weathering and chemistry of the waters. The clay minerals in the related soils from the watershed areas were also examined. It was found that the relative proportion of kaolinite to total clay minerals is a good measure for the degree of weathering of the area. The Ca concentrations of the stream waters are rather high in comparison with other cations such as Na, K and Mg, and are well correlated with HCO₃^-. These facts indicate that the Ca concentrations in the stream waters are largely controlled by dissolution of calcite that occurs as disseminated crystals in granites. The Ca/Na molar ratio in the waters decreases with increasing kaolinite/clay ratio in the related soils. The high Ca waters tend to occur in the fresh or weakly weathered areas, which is attributed to abundant calcite in these granites. By contrast, in the strongly weathered areas, where calcite had already been consumed via previous weathering processes, low Ca waters occur. The REE abundances, especially light REE, of the stream waters tend to be high in the strongly weathered watershed areas, and low in the fresh or weakly weathered watershed areas. This is attributed to either enhanced REE leaching from rocks in strongly weathered watershed areas due to larger solid surface areas or increased weathering of accessory minerals such as apatite. Interestingly, the magnitude of negative Ce anomalies appears to be inversely related to the degree of weathering. We suggest that the magnitude of Ce anomalies in those waters is largely controlled by mechanical filtration of CeO₂ during infiltration in micro-cracks or pores rather than by Eh-pH conditions.