Behaviour of rare earth elements and yttrium in ferromanganese concretions, gibbsite spots, and the surrounding terra rossa over dolomite during chemical weathering

The behaviour of rare earth elements (REEs) and yttrium (Y) during chemical weathering processes has been investigated on a 4.05 m thick terra rossa profile over dolomite on the Yunnan–Guizhou Plateau, China. In this profile, ferromanganese concretions and gibbsite spots coexist in the terra rossa solum. Analyses of REEs, Y, Zr, F, S, and TOC, as well as mineralogical studies, were conducted on a suite of ferromanganese concretions, gibbsite spots, terra rossas, dolomites, and insoluble residues from the underlying dolomite. These analyses helped us to understand the mobilization, redistribution, and fractionation of REEs and Y during chemical weathering. The REEs and Y are mobilized and redistributed during the terra rossa formation. REEs, except for Ce, are removed from the upper and middle profiles, transferred downwards, and then precipitated in the base profile, resulting in remarkable enrichment in the terra rossa near the weathering front. The significant increase of pH near the weathering front was responsible for REE (excluding Ce) and Y enrichment in the base profile. Y is quite mobile during extreme chemical weathering, and most of it was carried away from the profile. Because of very efficient oxidative fractionation of Ce, a significant positive Ce anomaly in terra rossa was found in the middle of the profile, whereas the terra rossa near the weathering front exhibited notably negative Ce anomalies. Moreover, the presence of cerianite and its content in the ferromanganese concretions can explain the markedly positive Ce anomaly and the variation of Ce/Ce* values with depth. In profile, these Ce anomalies are characteristic of the geochemical environment, especially the redox condition. Studies of REE distribution in the ferromanganese concretions, gibbsite spots, and surrounding terra rossa revealed that significant REE fractionation occurred. REEs and Y were preferentially concentrated in the ferromanganese concretions rather than in the gibbsite spots. The water–rock interaction resulted in M-type tetrad effects in some of the ferromanganese concretions, gibbsite spots, and terra rossa samples. In addition, the tetrad effect in terrestrial weathering processes likely played an important role in the fractionation of REEs and Y, such as Y–Ho, Sm–Nd, and the Eu anomaly change.