Secondary minerals in soils can record climatic changes affecting continental surfaces over geological times. Their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion/weathering regimes. In the present study, twenty kaolinite samples from two lateritic profiles of the Karnataka plateau, an intensively studied area in the southern India, have been dated using electron paramagnetic resonance (EPR) spectroscopy. Kaolinite ages vary between 0.229 ±0.24 Ma to 40.73 ±15.37 Ma. Four different groups of age can be identified with ages clustered around 1.0, 3.5, 9.0 and 39.0 Ma. These groups of age indicate local preferential weathering periods that coincide with distinct Indian climatic events described in independent studies, such as monsoon strengthening. Thus, regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles. These results confirm that despite their simple mineralogy, laterites can contain several relictual and coexisting generations of secondary minerals and that EPR dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods.

Ferralsols and Acrisols are major types of soils of the Amazon basin observed on various landform units comprising plateau surfaces, incised hills at their edges and slopes. The present study focuses on an Acrisol developed on plateaus surfaces from northwest Amazonia and a Ferralsol from the convex hills of the incised plateau edges. Local geomorphologic models of weathering covers suggest that Ferralsols are younger than plateau Acrisols but the absolute chronology of their formation is still lacking. This type of information is however critical to understand the evolution of northwest Amazon Basin landscapes and to identify its link with major climatic and geomorphologic events. In this paper, kaolinite-rich samples from soils and saprolites belonging to a transect in the São Gabriel da Cachoeira region (Amazon state, Brasil) are investigated. Based on their crystal-chemical characters, several types of kaolinites are identified. Using a previously developed methodology based on electron paramagnetic resonance (EPR) spectroscopy, crystallization ages are proposed for these different kaolinites. Saprolite kaolinites are dated from 6 to 3.6 Ma in the Acrisol profile and display significantly more recent ages (< 1 Ma) in the Ferralsol saprolite. Kaolinite from solum (soils horizons above the C horizon) display ages ranging from 2.5 to 1 Ma for both the Acrisol and Ferralsol. Three distinct weathering stages are thus unraveled by kaolinite dating. The Acrisol saprolite displays the older weathering stage preserved in the investigated soil sequence. It is followed by a single weathering stage leading to the formation of both soils. These two stages can be correlated to the formation of two paleosurfaces recognized at the scale of the South America subcontinent as the Miocene Vehlas and the Quaternary Paraguaçu surfaces, indicating that the Curicuriari profiles evolved during periods favorable to tropical weathering surfaces development in Amazonia. The last weathering stage corresponds to the saprolite formation in the Ferralsol profile, which is still developing under the present Amazonian climate. This still active, late 2 weathering stage is tentatively related to the more significant drainage and relief dissection occurring on the plateau edges.

International audienceInfrared field-based reflectance spectroscopy in the Visible-Near-Infrared-Shortwave Infrared (VIS-NIR-SWIR) domain is a useful tool in mining geology particularly efficient for investigating the clay mineralogy of alteration haloes around ore deposits. It is used as a routine technique for the basic identification, mapping and semi-quantification of clay mineral species. However, the use of this technique for prospecting in hypogene deposits at depth in intertropical areas is strongly limited because of the presence of a thick, kaolinite-rich lateritic cover. Due to the strong IR absorption of kaolinite and the overlapping of its IR bands with those of most of the phyllosilicates in the SWIR domain, the use of field based near-infrared spectroscopy does not permit efficient identification and mapping of the phyllosilicates inherited from hypogene alteration that persist in the saprolite of lateritic profiles. In this paper, we propose a methodology to enhance the detection and semi-quantification of hypogene phyllosilicate minerals in kaolinite-rich lateritic saprolites using calibration curves. Those curves are built from the NIR spectra of binary admixtures of kaolinite or smectite with muscovite, Fe-Mg chlorite, clinochlore or talc in different known proportions. For each admixture series, calibration curves were established, based on investigation of two regions of interest within the NIR domain (1350–1470 nm and 2080–2500 nm) using a field-based spectrometer. For each binary mixture series of phyllosilicates, the second derivative of the NIR spectra was used to enhance the detection of the diagnostic absorption bands of each type of phyllosilicate, and hence to optimize the calculation of the intensity ratios between the diagnostic bands of the phyllosilicate components as a function of their percentage in the mixture. In presence of large amounts of lateritic kaolinite, the detection limit of the major types of hypogene phyllosilicates has been found at ranges from 5 to 10 wt% of the total clay content using the second derivative of the NIR spectra acquired with a field-based spectrometer. Above these aforementioned limits of detection, the semi-quantitative data obtained by comparing the NIR reflectance spectra of natural samples with those of the calibration curves could permit to map hypogene alteration haloes directly from the lateritic sa-prolite. Finally the described approach has been successfully tested on natural samples from the skarn deposits of the Ity gold mine (Ivory Coast)