Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: The implication for the present dynamics of the soil covers of the humid tropical regions

The REE-Th weathering geochemistry and mineralogy has been investigated in a lateritic soil cover in relation to the close hydrographical system at Goyoum (East Cameroon). The stutied area, composed of gneissic hills covered by humid tropical forest, belongs to the Sanaga river basin and corresponds to a transition vegetation zone between rainforest and savannah. A representative soil catena was taken as a reference zone close to the Sanaga river. The gneissic parent-rock, soil, and groundwater samples have been cut off in pits in the lower part of the catena. Waters were also sampled both in brooks rich in organic matter and in the Sanaga river. All the waters were sampled seasonally during the rainy and dry periods. The water/rock interactions and the exportation of REE and Th have been discussed as a function of the evolution of specific ratios. Considering Th as the less mobile element during laterization, variation of parent rock (PR)-normalized ratios [(Ln/Th)PR] vs. depth show that the upper ferruginous horizons are depleted in REE while the basal saprolite presents accumulation zones which are differently located for Ce and the other LREEs. The HREEs are also depleted. The La accumulations are located near the saprolite/gneiss weathering front associated with negative Ce-anomalies (Ce/Ce* ∼ 0.4). The Ce accumulations inducing strong positive Ce-anomalies (Ce/Ce* ∼ 4) are located in the upper saprolite beneath the ferruginous horizons all along the soil catena. The secondary LREE-bearing minerals which reflect LREE fractionation during incipient weathering of the gneiss are hydrous phosphates (rhabdophane, LREEPO4,nH2O) and cerianite (CeO2) whose precipitation depends on the Eh-pH changes in the soil profiles. On the other hand, HREE depletion is controlled by the dissolution of xenotime, the major HREE bearer in the parent gneiss. In the <0.45 μm filtered acidic groundwater (pH ∼ 5.5) which have flooded the saprolite for a long residence time, Th contents are extremely low (0 < Th < 10 ppt) while LREE contents are high (90 < La < 450 ppt). In these filtered waters, Th seems to be strongly controlled by minerals (i. e., thorianite) which lead to its immobilization in the soil while LREE remain in the soluble and/or colloidal pools. Moreover, the groundwaters show significant negative Ce-anomalies throughout the year (0.1 < Ce/Ce* < 0.4. The mobilization of LREE, except redox-active Ce, results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative Ce-anomalies as indicated by the bulk soil analyses at the saprolite/gneiss weathering front. Therefore, the groundwaters have the signature of saprolitic materials. We propose that the strong positive Ce-anomalies of the upper saprolite result from ancient weathering processes. These processes would have led to much stronger Ce(III)/Ce(IV) dissolution/ reprecipitation conditions than those occurring at the present time in the saprolite. In the <0.45 μm filtered brook acidic blackwaters, LREE and Th contents are high (100 < Th < 400 ppt; < 400 La 1700 ppt). In this organic matter rich environment, LREE and Th are highly mobilized. The colloial pool controls half of the whole LREE-Th load. The geochemical signature of these waters is independent of the gneissic parent rock and soil LREE contents. There is no Ce-anomaly. This signature is similar to those of other organic-rich rivers draining humid tropical ecosystems in the world. In the 0.45 μm filtered Sanaga waters, LREE and Th contents are lower than in the brooks (15 < Th < 40 ppt; 40 < La < 300 ppt). These concentrations are dependent on the time and on the colloidal composition. However, 90% of these elements are mainly controlled by the suspended pool. Copyright © 1998 Elsevier Science Ltd.