Jean-Jacques Braun

Cerium anomalies in lateritic profiles

The REE geochemistry and mineralogy have been studied in four lateritic profiles, one derived from a syenite (Akongo, SW Cameroon), the others being developed on a gneissic basement and located along a soil toposequence (Goyoum, E Cameroon). There is a fractionation between LREE and HREE in the lateritic samples during weathering, the weathered residual products being enriched in LREE (from La to Eu) and depleted in HREE (from Gd to Lu); sampled waters are enriched in HREE in relation to the syenite host-rock. A positive Ce-anomaly has been found systematically at the top of the saprolite, beneath a zone of iron oxide accumulation in the laterite. Up to 2000 ppm Ce may be present. In the Akongo profile, cerianite, CeO2, is present as very fine coatings in non-ferruginous clayey domains. Primary REE-bearing accessory minerals are weathered at the bottom of the profile. Specifically, allanite and apatite are transformed into florencite and rhabdophane but these phases have no Ce-anomaly. All the data are interpreted as the result of the following processes: n1. n(1) REE leaching in a reducing environment. n n2. n(2) oxidation of Ce3+ to Ce4+ in an oxidizing environment. n n3. n(3) deposition of cerium as cerianite whereas the other REE remain in solution.

Mobilization and redistribution of REEs and thorium in a syenitic lateritic profile: A mass balance study

Abstract REE-Th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at Akongo in SW Cameroon. REE and Th mass balance calculations for the host-rock minerals show that at least 70% of the LREE s and 50% of the HREEs are contained in allanite, apatite, titanite, and epidote and at least 50% of the Th is controlled by the same accessory minerals which represent about 2 wt% of the unaltered syenite. These accessory phases are destroyed during the first stages of weathering causing most of the REEs and Th to be rapidly released into the soil. Comparison of the variation in the Zr, Ti, and Th content as a function of the apparent density of the different zones of the saprolite shows that Th is the least mobile element. The presence of secondary thorianite (ThO2), the etched surface on zircon grains, and the presence of Ti in secondary cerianite support this geochemical interpretation. The concentration of thorium was, therefore, chosen as invariant relative to the concentration of the other elements, especially the REEs, in mass balance calculations. Most of the REEs are leached in the iron-rich upper horizons (loose nodular horizon, iron crust, and top of mottled clay horizon). Where the groundwater table moves (saprolite and bottom of the mottled clay horizon), the REEs are fractionated and redistributed. There is a juxtaposition of leached and accumulation zones with precipitation of LREE aluminous hydrated phosphates. This study supports the existence of two different cycles for the redistribution of elements in the soil: 1. (1) as dissolved ions in the saprolite horizon, and 2. (2) as individual particles in the upper part of the profile.

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. n nConsidering 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. n nIn 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. n nIn 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.

Major and trace element abundances, and strontium isotopes in the Nyong basin rivers (Cameroon): constraints on chemical weathering processes and elements transport mechanisms in humid tropical environments

This paper aims to improve our understanding of chemical weathering processes and element transport mechanisms in the humid tropical environments. We studied the Nyong River basin (27,800 km2) located on the northwestern part (Ntem Complex) of the Congo craton (central Africa). The dissolved concentrations (i.e., <0.20 μm) of major and trace elements, dissolved organic carbon (DOC) and the 87Sr/86Sr ratios have been measured in more than 20 rivers draining watersheds with various surface areas (∼1 to 28,000 km2). All these rivers exhibit low major cations concentrations (i.e., Na, Mg, K, Ca) but high concentrations of some trace elements (Al, Fe, Th, Zr, Y, REE), silica, and DOC. The total dissolved load (TDS) is low (∼20 mg l−1) and dominated by silica and organic matter. The comparison of different watersheds shows us that chemical weathering is more efficient in the small unit-watersheds. All the sampled rivers exhibit a wide range of 87Sr/86Sr ratios and high Ca/Na ratio that can be explained by the heterogeneity of the crystalline rock constituting the cratonic basement. n nFour selected rivers (Mengong, Awout, Soo, and Nyong) having different drainage areas and hydrological parameters were sampled over a 2-year period. Collected data show that all these rivers present the same monthly seasonal variations, with higher concentrations during rainy season and lower concentrations during dry season. This implies that the weathering and transport mechanisms of small watersheds can be extended to the whole Nyong basin. In the small unit-watersheds, chemical weathering mainly occurs in swamp zones where mineral dissolution is enhanced by humic substances. These swamp zones constitute a pool of organic-rich water, which can be quickly mobilized during rain seasons. In these waters, DOC and insoluble element concentrations (e.g., Al, Fe, and Th) were strongly correlated which show the key role of organic colloidal matter in the transport of some insoluble elements. Some other relationships (Al vs. Fe, REE vs. Al) were also examined in order to get information about chemical weathering or element transport. n nBased on these data, it has been concluded that the chemical composition of these river waters is controlled by geomorphic and historic factors (e.g., thick cation-poor soil). In contrast, the present day climatic parameters (high rainfall and temperature) play a minor role in water chemistry regulation even though they are likely to enhance mineral dissolution. Even if organic matter favors mineral dissolution, chemical weathering in this area is low compared to other world regions, which suggest, on a global scale, a relatively small effect of these environments on the CO2 consumption.

Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France

ABSTRACT The thermal history of the Ardeche paleo-margin (southeastern France) has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation. These methods comprise Tmax and vitrinite reflectance measurements on organic matter, determination of the smectite content in mixed-layer illite/smectite, fluid-inclusion microthermometry in quartz, barite, dolomite, and anhydrite, and fission-track analyses in apatite. The Balazuc (BA1) drill hole, 1730 m deep, intersects mainly Jurassic carbonate rocks, Triassic sandstones, and sulfate-rich claystones and ends in Carboniferous rocks. The present bottom-hole temperature is 70°C. Other drill holes, less than 6 km away, were studied when necessary. One of the main objectives of this paper is to ssess the agreement between the different geothermometric methods, and to check their limitations and convergence when used on carbonate- and sandstone-dominated reservoirs. In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C. Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic. The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene. These paleotemperatures, much higher than the present ones, can be explained by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation implies a high average sedimentation rate during the Cretaceous, in agreement with data determined on a regional scale. A time-temperature evolution is proposed for the formations present in the Balazuc drill hole and the eroded sedimentary rocks. The application of the dual reaction model of the transformation of smectite to illite (Velde and Vasseur 1992) shows that the data are in good agreement with the kinetic model. However, the vitrinite reflectance variation trend with depth, predicted from the Burnham and Sweeney model (1989), is different from the data variation. No definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the BA1 drill hole.

Geochemical and mineralogical behavior of REE, Th and U in the Akongo lateritic profile (SW Cameroon)

Abstract The REE-U-Th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite (Akongo, SW Cameroon). Mass balance calculations have been done (1) in the syenite to determine the most important REE-U-Th bearing minerals and (2) in the soil profile to understand the movement of these elements. Thorium has been used as an immobile element for these calculations. The weathering of accessory minerals rich in REE and Th of the syenite (allanite, apatite, titanite and epidote) has been especially studied in the saprolite. Several REE-Th secondary minerals (florencite, rhabdophane, cerianite and thorianite), important to explain the fractionation of REE in supergene environments, have been found.

REY-Th-U Solute Dynamics in the Critical Zone: Combined Influence of Chemical Weathering, Atmospheric Deposit Leaching and Vegetation Cycling (Mule Hole Watershed, South India)

The source and proportion of REY, Th and U exported by groundwater and by the ephemeral stream along with the elemental proportions passing through vegetation have been assessed in the sub-humid tropical forested CZO of Mule Hole, Southern India. The study relies on a pluri-annual hydro-geochemical monitoring combined with a hydrological model. The significant difference between the soil input (SI) and output (SO) solute fluxes (mmol/km2/yr) of LREE (SI-SO = 13250-1500), HREE (1930-235), Th (64-12) and U (63-25) indicates a strong uptake by roots carried by canopy and forest floor processes. The contribution of atmospheric dust leaching can reach about 60% of LREE and 80% of HREE. At the watershed scale, the U solute flux exported by groundwater (180 mmol/km2/yr) mainly originates from the breakdown of primary U-bearing accessory minerals and dominates by a factor of 25 the stream flux. The precipitation of authigenic U bearing phases and adsorption onto Fe-oxides and oxyhydroxides plays a significant role for limiting the U mobility. In the groundwater, the plagioclase chemical weathering is efficiently traced by the positive Eu-anomaly. The very low (REY) to nil (Th) contents are explained by the precipitation of authigenic phases. In the stream flow, dominated by the overland flow (87% of the yearly stream flow), the solute exports (in mmol/km2/yr) of REY (1080 for LREE and 160 for HREE) and of Th (14) dominate those by groundwater. Their mobility is enhanced by chelation with organic ligands produced by forest floor and canopy processes.

Joint Use of Geophysical and Hydrological Methods to Characterize Structures and Flow Geometry in a Complex Aquifer

Characterizing the hydrogeological functioning of complex crystalline aquifers requires a precise assessment of their structural and hydrodynamic heterogeneities. This study shows how the joint use of TDEM, ERT and MRS enables to characterize both alterites and weathered-fissured zones. Moreover, with complementary data such as water table measurements and hydraulic tests, the characterization of the fractured zone is also possible. A conceptual hydrogeological model is then proposed from a case study in India, thanks to this joint use of geophysical and hydrological data.

Polar pesticide contamination of an urban and peri-urban tropical watershed affected by agricultural activities (Yaoundé, Center Region, Cameroon)

AbstractUrban agriculture is crucial to local populations, but the risk of it contaminating water has rarely been documented. The aim of this study was to assess pesticide contamination of surface waters from the Méfou watershed (Yaoundé, Cameroon) by 32 selected herbicides, fungicides, and insecticides (mainly polar) according to their local application, using both grab sampling and polar organic compounds integrative samplers (POCIS). Three sampling campaigns were conducted in the March/April and October/November 2015 and June/July 2016 rainy seasons in urban and peri-urban areas. The majority of the targeted compounds were detected. The quantification frequencies of eight pesticides were more than 20% with both POCIS and grab sampling, and that of diuron and atrazine reached 100%. Spatial differences in contamination were evidenced with higher contamination in urban than peri-urban rivers. In particular, diuron was identified as an urban contaminant of concern because its concentrations frequently exceeded the European water quality guideline of 0.200xa0μg/L in freshwater and may thus represent an ecological risk due to a risk quotient >u20091 for algae observed in 94% of grab samples. This study raises concerns about the impacts of urban agriculture on the quality of water resources and to a larger extent on the health of the inhabitants of cities in developing countries.n Graphical abstractᅟ