Oxygen and carbon isotope composition of structural carbonate in weathering apatites from laterites, southern Brazil and western Senegal |
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| Authors: | Jean-Pierre Girard Ren Flicoteaux Anne-Vronique Walter Samuel M Savin Daniel Nahon |
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| Institution: | Jean-Pierre Girard, René Flicoteaux, Anne-Véronique Walter, Samuel M. Savin,Daniel Nahon |
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| Abstract: | The oxygen (δ18Oc) and carbon (δ13Cc) isotope compositions of the structural carbonate group (CO3) in apatites from lateritic profiles were investigated. The weathering profiles, located in southern Brazil and in western Senegal, are developed on three different types of apatite-rich parent rock: carbonatite, metamorphosed marine phosphorite and sedimentary marine phosphorite. The parent rock apatites are of magmatic, hydrothermal, metamorphic and sedimentary origins. The in situ formation of apatite of weathering origin in the profiles is well documented petrographically and geochemically.The overall range of measured δ18Oc and δ13Cc values of apatites of weathering origin (22 to 27 SMOW for δ18Oc and −15 to −10 PDB for δ13Cc) is much smaller than the range of measured and/or published isotope compositions of parent rock apatites (4–35 for δ18Oc and −11 to +1 for δ13Cc). In any profile, the apatites of weathering origin can exhibit lower, similar or higher δ18Oc values than parent rock apatites. In contrast, their δ13Cc values are systematically and significantly lower than those of the parent rock apatites. Apatites formed as a result of weathering in laterites can therefore be readily distinguished from apatites of other origin on the basis of their isotope composition.Assuming that apatite CO3 fractionates O in a way similar to calcite CO3, the structural carbonate group of the apatites of weathering origin appears to form in approximate isotopic equilibrium with the weathering solutions. The very low δ13Cc values exhibited by these apatites indicate that the dominant sources of dissolved CO2 in the soil water are organic. The isotope composition of structural carbonate in apatite of weathering origin in lateritic profiles may provide useful information for paleoenvironmental studies. |
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