Sequential Extraction of U and Th Isotopes: Study of Their Intrinsic Distribution in Phosphate and Limestone Sedimentary Rock in Comparison with Black Shale

In this work, a study of the intrinsic behavior of U and Th isotopes in sedimentary rocks was performed by sequential extraction. The results for the phosphate layers and limestone interlayers of Oulad Abdoun Basin are compared to those obtained previously for Timahdit black Shale rocks. Characterization car-ried out by ICP-AES and SEM-EDX showed that apatite, calcite, dolomite, quartz and clays are the essential components of the inorganic matrix. The selective leached phases are treated radiochemically before measuring U and Th isotopes by alpha spectrometry. Among the results, it should be noted that the U isotopes were concentrated in an anaerobic environment. Humic acids that are the richest organic phase in U are responsible for the concentration of

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U,

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U in the carbonates and apatite. In the mineral phase,

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Th, of terrigenous origin is partitioned between the silicate and pyrite minerals.