Laser oxygen isotope analysis of weathering goethite from the lateritic profile of Yaou,French Guiana: paleoweathering and paleoclimatic implications |
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| Institution: | 1. Irish Centre for Research in Applied Geosciences, Department of Geology, Trinity College Dublin, Dublin, Ireland;2. Geological Survey Ireland, Beggars Bush, Dublin, Ireland;3. Irish Centre for Research in Applied Geosciences, School of Earth Sciences, University College Dublin, Belfield, Dublin, Ireland;1. Equipe Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F-75005 Paris, France;2. Observatoire Volcanologique et Sismologique de Guadeloupe, Institut de Physique du Globe de Paris, Le Houëlmont, 97113 Gourbeyre Guadeloupe, France;3. NASA Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA;4. NASA Astrobiology Institute, 4800 Oak Grove Drive, Pasadena, CA 91109, USA |
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| Abstract: | Oxygen isotopic compositions of weathering goethite pseudomorphs after pyrite in the lateritic profile of Yaou, French Guiana, were measured by laser fluorination. The laterite at Yaou is composed of a 25 m thick saprolite overlain by a 3 m thick latosol. Petrographic observations indicate that the pseudomorphic replacement of pyrite by goethite occurs at or near the weathering front and is complete. The goethite pseudomorphs are essentially devoid of Al and are progressively dissolved and partially replaced by a microcrystalline plasma of kaolinite and goethite in the upper horizons (latosol) of the profile.The laser technique used in this study permitted the determination of the δ18O value of individual grains of goethite and the investigation of grain-to-grain variation within a single sample (horizon) as well as vertical variation with depth. In the latosol, the goethite δ18O values range from 0.8 to 3.2%‰ and exhibit significant grain-to-grain and vertical variation. This most likely reflects undetected intragrain contamination with microcrystalline kaolinite. In the saprolite, the goethite δ18O values are consistent, ranging from 1.0 to 1.8%‰ between 3 and 18 m depth, and from 2.5 to 3.3‰ between 23 m depth and the weathering front at 28 m. Oxygen isotope compositions of present groundwater, “soil water” and rainfall at Yaou were also measured, and compared with calculated isotopic compositions for goethite-forming waters. Goethite pseudomorphs in the lower part of the saprolite are in isotopic equilibrium with present groundwater, indicating that they may be forming today or that they formed in the recent past under climatic conditions similar to present conditions. In contrast, goethite pseudomorphs found higher in the profile are not in isotopic equilibrium with present water but with a water depleted in18O by ≈ 1.50% relative to present groundwater. It is suggested that these low-δ18O pseudomorphs are probably older and formed under different paleoclimatic conditions, at a time when climatic regimes were possibly more monsoonal than today. This is in agreement with models of climatic evolution derived from pedological considerations.The results of this study suggest that goethite pseudomorphs in ancient saprolites may preserve their original O isotopic composition acquired at the weathering front, and may, therefore, be used as indicators of changes in weathering-climatic conditions during the evolution of a profile. |
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