Le cycle de l'osmium et du rhénium pendant l'érosion himalayenne

Abstract : This study allowed determination of the Os isotopic compositions of the three major Himalayan formations (TSS, HHC, LH), based on both source rocks and river sediments, which represent the erosional products of Himalayan erosion. The High Himalayan formations (TSS+HHC) have 187 0s/188 0s ratios (0.9 to 1.6) similar to that of average continental crust. In contras t, the LH includes very radiogenic Os isotopic ratios, found in black shales and some carbonates. River sediments are mixtures of three components: High Himalayan silicates, LH black shales and LH carbonates. Sediments from the Brahmaputra are less radiogenic than those from the Ganga, due to either the addition of an ophiolitic fraction or the absence of detrital carbonates in, the sediments. The origin of the radiogenic Os isotopic compositions in LH carbonates is related to Himalayan metamorphism which mobilized Re and radiogenic Os from black shales and redistributed it in carbonates. Alteration at low temperature was studied in soil profiles. The concentrations of Re and Os, as well as the Os isotopic ratios, do not vary with depth. Nevertheless, an important loss of Re and Os occurs during the earlier stages of alteration, during the passage from source rock to saprolith. Sediments from the Bengal Fan show that Himalayan erosion is the major source of detritic material, especially in the active fan. In the rest of the Bay of Bengal, the Himalayan source may be masked by secondary sources, such as the erosion of the Indo-Burman range or the Sri Lanka. There is no evidence for significant loss of Os during sediment transport or after deposition in the Fan. This implies that the contribution of Himalayan erosion to seawater is limited to that carried in dissolved form, which is not nearly sufficient to explain the increase of the 187 0s/188 0s seawater ratio during the past 16 Ma.
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Anne-Catherine Pierson Wickmann. Le cycle de l'osmium et du rhénium pendant l'érosion himalayenne. Sciences de la Terre. Institut National Polytechnique de Lorraine, 2000. Français. ⟨NNT : 2000INPL058N⟩. ⟨tel-01749943⟩

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