1 Introduction

Au sein des profils latéritiques typiques en Afrique, Australie et Inde, l'horizon tacheté constitue la transition entre les horizons d'altération et les horizons ferrugineux cuirassés et nodulaires superficiels [1,4,9–12,18]. L'altération fournit des volumes riches en kaolinite néoformée et d'autres riches en quartz hérité [17,18]. Dans les volumes riches en kaolinite, au sein de l'horizon tacheté, le quartz est dissous [11] et la kaolinite substituée par de l'hématite [2]. Ainsi s'individualisent des taches rouges ou violacées. Localement, en présence de gœ thite, se forment des taches jaunes. Dans les volumes riches en quartz, un lessivage différencie des taches grises [3]. Vers le haut des profils, les taches rouges ou violacées se concentrent et s'indurent pour former successivement la carapace, la cuirasse et les horizons nodulaires. L'horizon tacheté évolue en cuirasse ferrugineuse (pétroplinthite) par concentration du fer et induration. L'absence d'horizon cuirassé ou nodulaire au-dessus d'un horizon tacheté, décrite en Afrique, est le résultat d'une ablation de la cuirasse [5,8]. Contrairement au cas de l'Afrique, de vastes étendues au Brésil présentent un horizon tacheté peu profond (plinthite ou paraplinthite) et peu d'affleurements de cuirasse ferrugineuse [13,14,16]. La cuirasse a-t-elle disparu ou ne s'est-elle pas formée ? En contribuant à la connaissance des couvertures à latérite d'Amazonie, l'objectif est de montrer qu'un horizon tacheté peut aussi se former du fait d'une perte en fer.

2 Site et méthodes d'étude

3 Résultats

2. L'horizon tacheté a été observé en microscopie optique à différentes profondeurs. Les pores sont des fissures délimitant des polyèdres (Fig. 1A) et des chenaux. Les taches rouges sont localisées préférentiellement au centre des polyèdres. Les taches RS sont constituées d'un plasma ferrugineux isotrope. Les taches RV sont constituées de particules ferrugineuses (2 à 3 μm), distribuées au sein d'un plasma gris massépique entre des grains quartzeux (Fig. 1B). Les taches GC se localisent entre les taches rouges et la paroi des pores (Fig. 1A). Elles sont composées d'un plasma et de grains identiques aux précédents. Enfin, les taches JV sans extinction sont en imprégnation sur la paroi des pores et, plus rarement, sur le bord des taches rouges. De rares argilanes gris sont observés au sein des taches GC. Aucun ferrane n'est présent et les ferriargilanes sont très rares.

4. Les caractéristiques micromorphologiques étant semblables au sein de l'horizon tacheté, une lame mince représentative a été sélectionnée pour des analyses chimiques. Les principales variations concernent la teneur en Fe₂O₃ (3 à 90 %) et, corrélativement, la teneur en SiO₂ + Al₂O₃ (Tableau 2). Dans les taches RV (Figs. 2A et 2B), les particules ferrugineuses sont constituées de 55 à 70 % de Fe₂O₃, alors que le plasma gris sombre en contient 3 % au plus. Les taches RS sont composées de 70 à 90 % de Fe₂O₃ (Figs. 2A et 2D). Le plasma gris sombre des taches GC en comporte environ 2 % (Figs. 2E et 2F) et le plasma gris clair des taches JV de 15 à 25 %. Le rapport Si/Al varie de 1,1 à 1,3 et la teneur en K₂O de 1 à 3 %. Ces caractéristiques sont conformes à la minéralogie (kaolinite et illite). Le caractère circonscrit des particules ferrugineuses, très net au MEB, n'est pas rendu par les cartes en isovaleurs de Fe₂O₃ (Figs. 2A et 2B). Ceci est dû au fait que les volumes analysés pour les points contigus se chevauchent, car la maille est de 1 μm entre chaque point.

Table 2

Chemical composition of the spots: RV, light red; RS, dark red; GC, light gray; JV, yellow (measurements obtained by quantitative microprobe on thin section).

Composition chimique des taches rouge vif (RV), rouge sombre (RS), gris clair (GC) et jaune vif (JV). Analyses obtenues par microsonde quantitative sur lames minces.

Spot	Fe2O3	SiO2 + Al2O3	Si/Al	K2O
	g per 100 g	g per 100 g		g per 100 g
RV
– Fe-rich particles	55–70	30–45	1.1	1
– Dark gray plasma	3	95	1.1	2
RS	70–90	10–30	1.3	1
GC	2	95	1.2	3
JV	15–25	75–85	1.1	2

4 Discussion

5 Conclusion

Within the lateritic profiles that are typically thick and continuous in Africa, Australia and India, the mottled horizon constitutes the transition from weathered bedrock up to the lateritic duricrust and lateritic gravel [1,4,9–12,18]. The weathering process supplies kaolinite-rich volumes (in situ neoformation of kaolinite) and the other quartz-rich volumes (inherited from the bedrock) [17,18]. Two processes mix to form the mottled horizon. On the one hand, quartz is dissolved [11] in the kaolinite-rich volumes and kaolinite is substituted by iron sesquioxides, mainly hematite [2]. Iron enrichment occurs and forms red or purplish spots. Locally, the association of kaolinite and goethite causes yellow spots. On the other hand, a leaching in the quartz-rich volumes creates gray-coloured spots [3]. Towards the top of the lateritic profiles, the red or purplish spots concentrate to form successively the lateritic duricrust. The lateritic gravel results from the superficial weathering of the crust. The lateritic duricrust (petroplinthite) is considered as derived from the mottled horizon by iron enrichment and hardening. In Africa, the absence of lateritic duricrust or lateritic gravel over a mottled horizon is interpreted as the result of a dismantling of the duricrust and gravel [5,8]. Contrary to those in Africa, the laterite covers in Brazil exhibit large areas with mottled horizon at shallow depth (plinthite or paraplinthite) and few areas with superficial lateritic duricrust or lateritic gravel [13,14,16]. Did the duricrust and gravel disappear or did they not form there? The objective is to contribute to the knowledge of the laterite covers in the Amazon Basin and to show that mottled horizon can also form because of iron depletion.

The site [15] is located in the southern part of the Amazon Basin (8°18′S, 63°48′W, altitude 100 m), between the localities of Porto Velho (State of Rondonia) and Humaita (State of Amazonas) in Brazil. The climate is characterised by mean annual precipitation of 2250 mm and mean annual evapotranspiration of 1500 mm, with a dry season from June to July (monthly precipitation lower than 40 mm). The relief is dominated by undulated plateaus. The original vegetation consists of alternating savannah and open forest large areas [14]. Within the laterite cover, which developed from sediments of the Solimões group (Middle Pliocene–Upper Pleistocene), the predominant soil is considered to be a Plinthic Cambisol [6]. The groundwater table is close to the surface (1 m), except during the dry season (3 m).

The macroscopic observation of the soil [15] was carried out in a trench (3.5 m depth), situated on a high part of plateau. Undisturbed and disturbed samples were collected. The granulometrical, mineralogical and chemical compositions were determined [15]. Thin sections of the mottled horizon were observed using an optical microscope and a scanning electron microscope (SEM) in the backscattered electron mode. The images reveal the chemical composition of the groundmass on a gray scale. Small areas were selected for quantitative chemical measurement of the major components (Si, Al, Fe, Ti and K) by electron probe microanalysis [7]. For each area (20×20 μm), maps of the chemical composition were obtained by measuring the elements with a spacing of 1 μm between the points of measurement. The data acquired were interpolated by linear kriging.

Four horizons were identified from the bottom to the top of the profile: mottled horizon, intermediate horizon, subsuperficial red horizon and upper humus horizon. All the horizons exhibit silty–clayey texture. The mottled horizon is 2.5 m thick and exhibits contrasting spots, light red (RV) 10R 4/8 to 2.5YR 4/6 (all Munsell colours referred to moist soil), dark red (RS) 10R 3/6, light gray (GC) 10YR 7/2 and yellow (JV) 10YR 7/8. Red RV and RS spots formed a three-dimensional network (1–3 cm thick), usually with vertical elongation. GC spots also form a three-dimensional network, similar in size and imbricated with the red network. The red spots predominated between 3.5 and 1.1 m depth (RV + RS ≈60%, GC ≈30% and JV ≈10%), while the gray spots predominated between 1.5 and 1.1 m (GC ≈60%, RV + RS ≈30% and JV ≈10%). The weak porosity consisted of fissural and tubular voids (diameter 0.5–1 mm) and was located within GC spots. The transition, between 1.1 and 0.6 m depth, from the mottled horizon to the upper horizon was characterised by the gradual disappearance of the mottled facies. The intermediate horizon was composed of a light yellowish–brown 10YR 6/4 matrix, in which abundant red 2.5YR 4/8 spots (soft, diffuse limit and millimetric to centimetric size) and ferruginous nodules (10R 4/6, 2 mm in diameter, soft, contrasted limit) were present. The spots decreased gradually in size and in proportion closer to the surface, and their colour became less lively and their outlines less defined than in the mottled horizon. The subsuperficial horizon (30 cm thick) was homogeneous red 2.5YR 5/6 and contained ferruginous nodules (similar to those of the underlying horizon). The upper humus horizon (30 cm thick) exhibited a brown colour varying from the bottom (5YR 4/4) to the top (10YR 3.5/2).

The granulometrical study indicated a large clay fraction (Table 1). In the area, clay fraction was composed of kaolinite and illite; the subsuperficial red horizon differed from the others by the additional presence of Al-vermiculite [15]. The chemical composition was variable within the mottled horizon. The SiO₂ content ranged from 76 to 63%, the Al₂O₃ content from 15 to 22% and the Fe₂O₃ content from 7 to 12%. However, there was little variation between the top of the mottled horizon and the upper red horizon (Table 1).

Thin sections of the mottled horizon (250, 200 and 150 cm depth) were observed by optical microscopy. The pores were composed of planar voids, which delimited blocks (2–4 mm in size) and channels (millimetric diameter) resulting from biological activity. The skeleton (essentially quartz) was randomly distributed within the RV, GC and JV spots. Red spots were mostly located in the centre of the blocks (Fig. 1A). RS spots (continuous on surfaces from 70 to 700 μm) consisted of undifferentiated isotropic ferruginous plasma. RV spots (tens or hundreds of microns) consisted of well-differentiated ferruginous particles (2–3 μm in diameter), which were randomly distributed within gray masepic plasma between the skeleton grains (Fig. 1B). GC spots were located between red spots and the walls of the voids (Fig. 1A). They consisted of a gray masepic plasma and skeleton grains identical to those described above. JV spots impregnated the walls of planar or biologic voids and more rarely the border of RS or RV spots. Gray argillans within GC spots and ferri-argillans were rarely observed. No ferran was present.

The SEM observations showed that the RV spots consisted of white and dense ferruginous particles, 2–3 μm in diameter, rounded or subangular, mostly isolated and spaced 3–10 μm apart (Figs. 1C–1F). They contrasted with the dark gray surrounding plasma, which consisted of undifferentiated components (×3000) and well-differentiated clay particles (probably of kaolinite and illite), without differentiated ferruginous components (Figs. 1C and 1E). The centre of RS spots consisted of white and very dense ferruginous components, with or without individualization of ferruginous particles, without dark gray plasma and without skeleton grains (Figs. 1G and 1H). On the border of RS spots, ferruginous particles (0.5–1 μm) occurred within dark gray plasma. They became less dense with distance from the centre of the spots. GC spots and JV spots were systematically associated with the voids (Figs. 1C and 1D). They consisted of gray plasma, without differentiated ferruginous components, but appearing lighter in JV spots than in the GC and RV spots (Figs. 1C and 1D).

The micromorphological characteristics being similar within the mottled horizon, a representative thin section was selected for chemical microanalysis. The main variations were related to the content of Fe₂O₃ (3–90%) and the correlated content of SiO₂ + Al₂O₃ (Table 2). The iron content varied in proportion to the redness of the various spots. In RV spots (Figs. 2A and 2B), the ferruginous particles consisted of 55 to 70% Fe₂O₃, while the gray dark plasma contained at most 3% Fe₂O₃. In RS spots the Fe₂O₃ content ranged from 70 to 90% (Figs. 2C and 2D). The gray dark plasma in the GC spots contained about 2% Fe₂O₃ (Figs. 2E and 2F) and the light gray plasma of JV spots from 15 to 25% Fe₂O₃. The Si/Al atomic ratio ranged from 1.1 to 1.3 and the content of K₂O from 1 to 3%. These characteristics are consistent with the kaolinite and illite composition of the plasma. The predominantly isolated character of the ferruginous particles that appears in the SEM images was not reflected in the maps of the Fe₂O₃ contents. This is because the distance between the points of analysis was 1 μm and the volumes analysed for adjoining points overlapped one another.

The mottled horizon within typical lateritic profiles constitutes the transition from the weathered bedrock up to the lateritic duricrust. Schematically, red spots and yellow spots differentiated because of the dissolution of quartz and the substitution of kaolinite by iron sesquioxides in kaolinite-rich volumes (inherited from the weathering) and gray spots resulted from the leaching of clay in quartz-rich volumes (also inherited). The lateritic duricrusts are considered as derived from mottled horizon by iron concentration and hardening.

In the case studied, gray and yellow spots were located near the fissures and the channels, while red spots were systematically away from the voids. Gray spots and yellow spots consisted of a clayey plasma (kaolinite and illite) associated with skeleton grains (quartz). The plasma of gray spots appeared by SEM dark gray, without distinctive iron component, whereas the iron content was very low (2%). The plasma of yellow spots that appeared also without distinctive iron component was lighter gray, with a higher iron content (15–25%) than in gray spots. The light red spots consisted of clayey plasma and skeleton grains similar to those of gray spots. Nevertheless, mainly contrasted and isolated Fe-rich particles were set in the clayey plasma. The iron content was very low (2%) in the clayey plasma and very high (55–70%) in the Fe-rich particles. The dark red spots consisted of dense concentration of Fe-rich particles, the clayey plasma being present only in the border where the Fe-rich particles were separated.

The processes generally involved in the spot differentiation did not explain the iron segregation (isolated or concentrated Fe-rich particles) in the red spots away from the voids and the relatively low iron content (absence of such Fe-rich particles) in the gray spots or yellow spots closed to the voids. This pattern does not result from iron concentration or depletion process according to the present pedological conditions. Therefore, we assume that it results from previous conditions. In the case studied, an initial horizon was uniformly composed of clayey plasma, skeleton grains and Fe-rich particles (isolated or concentrated). The current fissures and channels developed later. Currently, the red spots correspond to relict feature of the initial horizon. The transition from the red spots to the gray spots corresponds to the disappearance of the Fe-rich particles and the preservation of the clayey plasma with skeleton grains. The planar voids and the channel voids are systematically associated with gray spots or yellow spots, which result from depletion and transport of iron, respectively. The features of Fe-accumulation were absent in porosity (only argillans were rarely present in gray spots). Probably, the most of the dissolved iron was exported through pores during the periods of saturation of the mottled horizon by the groundwater table.

The mottled horizon has been developed within a profile less weathered (presence of illite and vermiculite) than the typical lateritic profiles and without duricrust. The evolution of the mottled horizon is complex. The light red spots consisted of clayey plasma in which isolated Fe-rich particles were set and the dark red spots consisted of concentrations of Fe-rich particles. This pattern is inherited from previous evolution, characterised by iron segregation. Gray and yellow spots, which are systematically associated with the present porosity, correspond to the disappearance of Fe-rich particles. We assume that the predominant process responsible for the mottled differentiation implies iron depletion. Therefore, the absence of Fe-accumulation explains why the evolution of the mottled horizon has not resulted in a lateritic duricrust.