The physical properties of Brazilian Oxisols are closely related to the development of their microstructure, which typically consists of stable microaggregates smaller than 1 mm. There is no model available to predict changes in microstructure in Oxisols. The objective of this work was to relate the proportion of microaggregates to the bulk density (D b) in the soil studied. Five sites of a typic Haplustox under native vegetation (two sites) and pasture (three sites) were sampled. Soil bulk density, sand, silt, and clay content and aggregate size distribution were measured from the surface to 1.6 m deep in increments of 0.1 m. Thin sections were prepared from undisturbed samples collected in duplicate from 0-0.1 m, 0.3-0.4 m, 0.8-0.9 m and 1.5-1.6 m depth, and backscattered electron scanning images (BESI) were generated. Clay content ranged from 672 to 798 g kg-1 and bulk density between 0.87 and 1.18 g cm-3 among the 80 samples studied. D b was poorly correlated with clay content (R² = 0.358) and at any depth was not significantly smaller under native vegetation than under pasture. Visual assessment of BESI revealed that soil material corresponded to either microaggregates (< 0.1 mm) in loose arrangement or to much larger aggregates. Quantification of BESI from the deepest sampling depth of all soils showed that 96.2 and 95.7 % of microaggregates were < 0.8 mm with 73.2 and 95.7 % between 0.1 and 0.5 mm under native vegetation and pasture, respectively. The mass proportion of microaggregates can be estimated using the < 0.84 mm soil material that is obtained by dry sieving (<0.84). Finally, our results showed that <0.84 varied with D b. Linear regression coefficients were calculated for the relationship between <0.84 and the reciprocal of bulk density (1/ D b) (<0.84 = 1.97 (1/ D b)-1.52, R² = 0.82), assuming no interaction between microaggregates and macroaggregates, the porosity of these two structural types was estimated as 0.71 and 0.51, respectively.

Geochronological, mineralogical, and geochemical analyses have been focussed on the Mont-Lozère-Borne plutonic complex and surrounding rocks (Cévennes, French Massif Central) in which B-W-Sn and As-Au-Sb mineralization is encountered. Two main results are highlighted: (1) the existence of a 301-306 Ma magmatohydrothermal event unrelated to the emplacement of the Pont-de-Montvert-Borne plutonic body at 316 Ma; (2) the magmatic and hydrothermal features are strongly associated, both in time and in space, thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the French Massif Central. We also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results. This study demonstrates that the application of the O and H isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care. Finally, a model of the tectonic-magmatic-hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios. The first one implies the existence of two different hydrothermal/ mineralizing events (Bo-W-Sn and As-Au-Sb ones). The second one suggests the same source for all hydrothermal and mineralized structures.

Subrounded microaggregates 100 to 500 µm in diameter were collected in a Ferralsol located in the state of Goiàs. Observation of these microaggregates in dry condition in scanning electron microscopy showed morphological characteristics that are representative of microaggregates described earlier in many Ferralsols. The shrinkage for drying of initially water saturated microaggregates was studied using a environmental scanning electron microscope. Results showed very small shrinkage for the microaggregates studied as usually considered. However the shrinkage varies since the volume of dried microaggregates was found to be 93 to 99 % of their volume when they are saturated. Cerrados / environmental scanning electron microscope / microped / structure Résumé-Analyse directe du retrait de microagrégats durant la dessiccation : Application à des microagrégats d'un Ferralsol argileux brésilien. Des microagrégats de forme subarrondie et de 100 à 500 µm de diamètre ont été prélevés dans un Ferralsol localisé dans l'état du Goiàs. L'observation à l'état sec de ces microagrégats en microscopie