Daniel Nahon

The effect of organic matter on chemical weathering. Study of a small tropical watershed: Nsimi-Zoetele site, Cameroon

Abstract The effect of organic matter during soil/water interaction is still a debated issue on the controls of chemical weathering in a tropical environment. In order to study this effect in detail, we focused on the weathering processes occurring in a small tropical watershed (Nsimi-Zoetele, South Cameroon). This site offers an unique opportunity to study weathering mechanisms in a lateritic system within a small basin by coupling soil and water chemistry. The lateritic cover in this site can reach up to 40 m in depth and show two pedological distinct zones: unsaturated slope soils on the hills and/or elevated areas; and water-saturated soils in the swamp zone which represent 20% of the basin surface. The study present chemical analysis performed on water samples collected monthly from different localities between 1994–1997 and on soil samples taken during a well drilling in December 1997. The results suggest the existence of chemical and spatial heterogeneities of waters in the basin: colored waters flooding the swamp zone have much higher concentrations of both organic matter (i.e., DOC) and inorganic ions (e.g., Ca, Mg, Al, Fe, Th, Zr) than those from springs and groundwater from the hills. Nevertheless, these organic-rich waters present cation concentrations (Na, Ca, Mg, K) which are among the lowest compared to that of most world rivers. The main minerals in the soils are secondary kaolinite, iron oxi-hydroxides, quartz, and accessory minerals (e.g., zircon, rutile). We mainly focused on the mineralogical and geochemical study of the swamp zone soils and showed through SEM observations the textural characterization of weathered minerals such as kaolinite, zircon, rutile, and the secondary recrystallization of kaolinite microcrystals within the soil profile. Water chemistry and mineralogical observations suggest that hydromorphic soils of the swamp zone are responsible for almost all chemical weathering in the basin. Thus, in order to explain the increase of element concentration in the organic-rich waters, we suggest that organic acids enhance dissolution of minerals such as kaolinite, goethite, and zircon and also favors the transport of insoluble elements such as Al, Fe, Ti, Zr, and REE by chemical complexation. SiO2(aq) concentrations in these waters are above saturation with respect to quartz. Dissolution of phytholithes (amorphous silica) may be responsible for this relatively high SiO2(aq.) concentration. Al/Mg ratios obtained for the soil and the Mengong river waters show that a significant amount of Al does not leave the system due to kaolinite recrystallisation in the swamp zone soils. Geochemical data obtained for this watershed show the important contribution of vegetation and organic matter on chemical weathering in the swamp zone. Quantitatively we propose that the increasing amount in total dissolved solid (TDS) due to organic matter and vegetation effect is about 35%. In summary, this interaction between soils and waters occurs mostly in soils that are very depleted in soluble elements. Thus, the low concentration of major elements in these water is a direct consequence of the depleted nature of the soils.

Water-rock interactions in tropical catchments: field rates of weathering and biomass impact

The knowledge of dissolution and precipitation rates of rocks and minerals is important to understand the effect of natural as well as human influences on the soil system. The combination of mass-balance calculations and water sample geochemistry allows us the computation of weathering rates for small watersheds. The basaltic area of Ribeirao Preto (Parana, Brazil) provides an opportunity to study solute acquisition by meteoric waters in basaltic terranes, to quantify the water-basalt interaction, to address the impact of biomass on the weathering and to estimate the age of the weathering processes affecting the basalts under humid tropical climate conditions. The mass-balance approach is extended to Si and trace elements as well as neoformed minerals. The computations are showing that the transformation rates calculated with a biomass factor are 1.3–5 times faster than the rates achieved for the model not taking into account the biomass. The rates of release of Si (log k, k in mol m−2s−1 Si) computed for augite and labradorite have the same magnitude: −12.6 and are two orders of magnitude smaller than the experimental rates. The rate of precipitation of kaolinite (log k, k in mol m−2s−1 Si) is − 15.2. The mean rate of lowering of the weathering front has a value of around 40 ± 10 mm kyr−1 under humid tropical climate. A mean age of 700 ± 200 kyr is calculated for the weathering processes affecting the basalts of the Parana basin.

A geochronological 40Ar/39Ar and 87Rb/87Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil

Kue5f8Mn oxides of hollandite group minerals such as cryptomelane (K1–2(Mn3+, Mn4+)8O16nH2O) are often precipitated authigenically in weathering profiles. The presence of structural K allows these minerals to be dated by the Kue5f8Ar and 40Ar/39Ar methods, making it possible to study the progression of oxidation fronts during weathering processes. Within the context of a recent 40Ar/39Ar study of cryptomelane from the Azul Mn deposit in the Carajas region (Amazonia, Brazil), Vasconcelos et al. (1994) defined three age clusters (65–69, 51–56, and 40–43 Ma) and proposed that they correspond to the episodic precipitation of the three generations of Mn oxide that have been identified in the deposit (Beauvais et al., 1987). n nWe performed a laser probe 40Ar/39Ar and 87Rb/87Sr study on new samples from the same Mn deposit. Our 40Ar/39Ar data confirm that cryptomelane is a suitable mineral for 40Ar/39Ar dating, although in some cases we clearly identify the existence of 39Ar recoil effects. Although the corresponding age spectra are generally strongly disturbed, our results also confirm that the earliest cryptomelane generation is of Upper Cretaceous-Paleocene age. We obtained good plateau ages from veins and concretions of the second cryptomelane generation. Some of these results allow definition of a well-constrained age cluster at 46.7–48.1 Ma not observed by Vasconcelos et al. (1994). A petrographic study confirms that none of the samples analyzed in the present study contained material associated with the third generation of cryptomelane. We propose that these new results support the idea of a more or less continuous crystallization of Kue5f8Mn oxides, mainly constrained by local factors, rather than the model advanced by Vasconcelos et al. (1994), which suggests that each cryptomelane generation corresponds to distinct weathering events related to global climatic changes. n n87Sr/86Sr data show large variations, clearly inherited from the 2.1 Ga parent rock of the Mn protore. The Rb/Sr results demonstrate that minimum fractionation occurs during cryptomelane crystallization, except for the latest generation, which is depleted in Sr. This precludes use of the Rb/Sr radiochronometer for dating secondary Mn oxides in laterites.

Dynamic model of the genesis of calcretes replacing silicate rocks in semi-arid regions

Abstract In both pedogenic and groundwater calcretes, calcium carbonate precipitates in voids, or displacing other grains, or replacing underlying parent silicates. Replacement textures are widespread in pedogenic calcrete. Many calcretes also contain magnesium layer silicates and minor chert. We present a reaction-transport model that accounts for the genesis of replacement in calcretes and for their mineralogy. Replacement is difficult to account for geochemically because it requires simultaneous removal of large amounts of silicates and import of also large amounts of CaCO 3 . In the model the genesis of replacement is directly related to seasonally alternating dry-wet climates and to appropriate groundwater (or circulating soil water) compositions. In a dry season, water evaporation causes CaCO 3 and sepiolite (or attapulgite) to precipitate. If groundwater contains enough Mg 2+ , sepiolite precipitation by the chemical-divide mechanism depletes SiO 2 (aq), resulting in the dissolution of parent silicates. In the following wet season, sepiolite dissolves fast, and silica and cations are flushed away by rainwater, making room for CaCO 3 precipitation in the next dry season. As climate cycles repeat, CaCO 3 is accumulated and silicates are removed. The sepiolite (or attapulgite, or Mg-smectite) serves as a temporary storage of silica between seasons. If the groundwater contains too little aqueous Mg then the model predicts growth of calcium carbonate without removing silicates, thus producing void filling and or displacive textures instead of replacement. The model consists of a set of nonlinear partial differential equations taking account of mass conservation, dispersion, advection, rainwater infiltration, evaporation, and the kinetics of mineral reactions. The hydrodynamics of unsaturated media is applied in determining water flow in calcrete profiles. Wet/dry seasonal changes are incorporated by alternating the upper boundary conditions. The model successfully produces the mineral and textural zonation observed in many calcretes (namely, at the bottom of the profile the parent rock is first replaced by sepiolite, only part of which is in turn replaced by calcite, whereas at the top of the calcrete the sepiolite is itself completely replaced by the calcite, which appears to “directly” replace the parent rock). The model can produce calcrete near the Earths surface well above the water table. Low P CO 2 , intensive evaporation, and long dry seasons all are predicted to produce thicker calcretes. Calcretes constitute an effective geochemical tool that, via replacement, removes parent silicate rocks and shapes the landscape of semi-arid countries. The model herein provides a mechanism that accounts for the efficiency of replacement in removing silicates.

Isotopic tracers of the origin of Ca in a carbonate crust from the Middle Atlas, Morocco

Abstract Carbonate crusts developed on basalt were studied at Tazouta in the Moroccan Middle Atlas. The chosen representative weathering profile is 2.8 m thick and is composed from base to top of a fresh basalt overlain by a coherent saprolite, a friable saprolite and a surface soil. Carbonate crust genesis occurs by replacement of primary and secondary clay minerals by low Mg calcite. The 14 C dates of the calcite range from 15 to 14 ka in the coherent saprolite, and from 12 to 8.5 ka in the friable saprolite. Smectitic weathering and replacement of Si-bearing minerals by carbonate are coupled processes leading to a strong calcium net gain (300 kg/m 3 ) within the friable saprolite. The low weathering rate of basalt compared to the calcrete genesis rate, and the high mobility of calcium in supergene weathering mantles, limit the basaltic Ca contribution to calcrete genesis. The contribution of the Triassic sediment is also limited by the mineralogical unavailability of Ca. The 87 Sr/ 86 Sr ratios confirm the low contribution of basalt and indicate an external origin of Ca. The Sr isotope ratios of calcrete are different from those of the adjacent Liassic carbonates and are close to rainfall values. The carbon isotope compositions of carbonates (−8‰ to −9‰ vs. PDB) imply a significant contribution of C3-dominated organic carbon during carbonate precipitation. Oxygen isotopic compositions of carbonates (average −5.24‰ vs. PDB), show that carbonates are 18 O-enriched compared to meteoric waters (−4‰ to −3.5‰ vs. SMOW).

Pseudomorphic replacement versus dilation in laterites: petrographic evidence, mechanisms, and consequences for modelling

The calculation of the mass transfer attending lateritic weathering was carried out by Brimhall et al. in 1991 by an approach that assumes immobility of one or another element, especially Zr. This calculation is flawed because: (1) Zr is mobile (as evidenced by etch pits in weathered zircons and by Eh-pH diagrams), as are Fe, Al, Ti, and others; (2) homogeneity of parent rocks, also required by that approach, is difficult to prove, and unlikely because of self-organization; (3) lateral fluxes, which must be neglected for the approach to work, are in fact inherent in weathering because of the infiltration instability; (4) the dilations calculated with this approach are inconsistent with long-known petrographie and field evidence of constant-bulk-volume evolution; (5) that evidence has been mischaracterized as assumption; and (6) the driving force hypothesized to cause the calculated expansion of some saprolitic laterites is contrary to reason. n nBetter calculations of mass transfer can be carried out on the basis of constant bulk volume framework which is effected in every tropical saprolitic profile by partial pseudomorphic replacement of parent mineral grains by oxides, oxyhydroxides, or kaolinite. Petrographie evidence of widespread replacement in laterites is conclusive. The partial replacement of each parent grain is driven by congruent dissolution of parts of other parent grains slightly above. Bulk-volume-preserving pseudomorphic replacement is produced by the grain/grain stress generated by crystal growth in a rigid rock. This stress requires the growth rate of the replacing grain and the dissolution rate of the replaced grain to be always equal to each other. Replacement (also widespread in diagenesis, metamorphism, and metasomatism) has significant consequences and implications for the modelling of mass transfer in weathering.

Rare-earth elements and isotopes (Sr, Nd, O, C) in minerals from the Juquiá carbonatite (Brazil): tracers of a multistage evolution

Three successive stages of crystallization (magmatic, late-magmatic and hydrothermal) were petrographically, mineralogically and chemically distinguished in the Juquia carbonatite (southern Brazil). The first stage corresponds to crystallization of magmatic Fe-dolomite and hydroxy-fluorapatite, with phlogopite micas and magnetite as accessories. The second stage is characterized by the formation of late-magmatic Na-rich carbonate-fluorapatite, followed by the crystallization of Sr-rich fluorapatite. Veins of barite represent the hydrothermal third stage of crystallization. Post-magmatic mineralizations induced re-equilibration processes which also influenced the geochemical characteristics of the carbonatite. The greatest changes are observed for trace elements (Na, Sr, Ba), REE, and 87Sr86Sr ratios. Isotopic ratios δ18O, δ13C and 143Nd44Nd values clearly support the magmatic origin of the two first mineralization stages. Evolution of 143Nd144Nd and 87Sr86Sr isotopic ratios during the post-magmatic processes indicates the possible participation of either some exogenic fluid, or batches of magma, with distinct compositions or more evolved fluids during crystallization of late-magmatic minerals. REE distributions show a progressive MREE to HREE enrichment, compared to LREE, along the three stages of mineralization. This evolution is explained as the result of preferential complexation of HREE in the last fluids in the form of carbonate, fluoride, phosphate or multiple complexes.

Diagenesis of the upper Proterozoic siliciclastic sediments of the Taoudeni Basin (West Africa) and relation to diabase emplacement

ABSTRACT The predominantly siliciclastic Upper Proterozoic sediments of the Taoudeni basin have been buried to depths of 2.5 to 3 km. In a normal geothermal gradient 25°C/km), maximum burial temperatures of the section studied would be no higher than 100°C. However, detailed petrographic and fluid inclusion data of sandstones and shales from three areas in Mauritania and Mall along the western side of the basin indicate two major diagenetic events: an earlier low temperature (< 70°C) quartz cementation of the sandstones and a later high temperature (135-170°C diagenesis that affected both shales and sandstones. These latter temperatures are too high to have occurred simply in response to burial. The sediments are cut by numerous Jurassic diabase intrusions, which crop out es ecially commonly in the south and the northeast. The emplacement of these diabases was related to the opening of the South Atlantic ocean. Contact metamorphism and hydrothermal alteration are visible near intrusive contacts. In western Mall (southern part of basin) where the largest exposed sill in the basin occurs, at 50 to 60 km from any outcropping massive diabase, the late diagenetic paragenesis consists of micas, illite, chlorite, kaolinite, siderite and ankerite. High-temperature diagenesis is also observed in Adrar of Mauritania (northwestern part of basin) where outcropping diabases are extremely rare. There, calcite cement is associated with illite and chlorite. Fluid inclusions indicate formation temperatures of 135°C for the ankerite and 170°C for the calcite. Thus, in spite of a thin sedimentary pile, the Taoudeni basin experienced a high-temperature diagenesis. We propose that this resulted from circulation of hot fluids, generated during the diabase emplacement, over extensive areas within the basin.

Diagenesis of Cretaceous Sandstone Reservoirs of the South Gabon Rift Basin, West Africa: Mineralogy, Mass Transfer, and Thermal Evolution

ABSTRACT The off-shore Gabon basin contains large hydrocarbon reserves in Lower Cretaceous argillaceous sandstones deposited in the rifting stage of the opening of the South Atlantic. The study area (5 10 km) includes a N30°E-trending fault system a few kilometers wide. In the rocks of this fault system, the diagenetic mineralogy consists of early siderite and abundant calcite cements, followed by late quartz, minor dolomite, and chlorite. The abundant calcite cement occludes most of the porosity. In the region adjacent to the fault system, the diagenetic mineralogy consists of early siderite and minor calcite, followed by late quartz, abundant dolomite, and kaolinite. Amounts of diagenet c cement in both regions range from ten to more than forty percent of the bulk rock volume. Mass balance based on petrographic evidence and point-counting indicates that such quantities of cement cannot have been generated locally, which thus implies considerable large-scale mass transfer. 18O values for coexisting (though not clearly cogenetic) dolomite and kaolinite from the kaolinite zone suggest temperatures < 100°C in late diagenesis. Published evidence from fission tracks in apatites and from fluid-inclusion homogenization temperatures indicates that the temperature during early diagenesis ( 130-120 Ma) was 200-250°C or even higher. We hypothesize that both the required diagenetic mass transfer and the observed distribution of diagenetic minerals were brought about by hot water flow. This circulation, perhaps convective, was driven by the high early geothermal gradients directly related to the opening of the South Atlantic, and was especially focused along the tensional fracture systems involved in the rifting. The abundant early calcite cement that occurs in the fracture system within the study area represents the diagenetic mineral signature of the high geothermal gradient that prevailed early in the history of the Gabon rift basin.

Laterites and Bauxites

The term «bauxite» was introduced by Berthier (1821) and refers to rock samples belonging to the laterite type. Lateritic bauxites have been studied by many, mainly by prospection and exploration geologists, and by geologists, soil scientists and geomorphologists interested in the mechanisms of bauxite formation and evolution. Many overviews have been published on the topic during this century (Lacroix 1913; Harrasowitz 1926; Harrison 1934; Patterson 1967; Valeton 1972; Bardossy and Aleva 1990). It is now well known that bauxites and laterites result from pedogenic processes, i.e. from weathering of a large variety of sedi-mentary, metamorphic and igneous rocks, under tropical and humid climates (Bocquier et al. 1984).