Negative cerium anomalies in the saprolite zone of serpentinite lateritic profiles in the Lomié ultramafic complex,South-East Cameroon

Strong negative cerium anomalies are developed in the saprolite zone of two serpentinite lateritic profiles in the Mada region of the Kongo–Nkamouma massif in the Lomié ultramafic complex (South-East Cameroon).The total lanthanide contents increase strongly from the parent rock (1.328 ppm) to the weathered materials (ranging from 74.32 to 742.18 ppm); the highest value is observed in the black nodules from the western weathering profile and the lowest one in the top of the clayey surface soil from the same profile. The lanthanide contents, except cerium, are highest in the saprolite and decrease along the profile. The light REE contents are very high compared to those of the heavy REE (LREE/HREE ranging from 3.21 to 44.37). The lanthanides normalized with respect to the parent rock reveal: (i) strong negative Ce anomalies with [Ce/Ce¹] ranging from 0.006 to 0.680 in the saprolite zone; (ii) strong positive Ce anomalies with [Ce/Ce¹] ranging from 1.23 to 23.96 from the top of the saprolite to the clayey surface horizon; (iii) positive Eu anomalies with [Eu/Eu¹] ranging from 2.09 to 2.41 in all the weathered materials.Mass balance evaluation shows that, except cerium, lanthanides have been highly accumulated in the saprolite zone and moderately concentrated in the upper part of both profiles. Cerium has been highly accumulated in the nodules of the West Mada profile. The presence of negative Ce anomalies is confirmed by its low degree of accumulation whereas the positive ones are related to its high degree of accumulation.     

Rôle des héritages latéritiques et du niveau des nappes dans l'évolution des modelés et des sols en zone intertropicale forestière humide

Analysis of landscapes and soil covers from the outer to the inner part of a $75\text{km}\times 60\text{km}$ watershed in the South-Cameroon rainforest zone, characterized by important remnants of inherited ferricretes and stable hydrostatic base levels, indicates that the development of present landforms has not involved significant erosion by slope retreat because of the relative preservation of downslope residual ferricrete and that soil covers have vertically decayed under in situ geochemical process. To cite this article: D. Bitom et al., C. R. Geoscience 336 (2004).