The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays |
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| Authors: | Norbert Clauer Michel Hoffert Anne-Marie Karpoff |
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| Institution: | Centre de Sédimentologie et Géochimie de la Surface, Institut de Géologie de l''Université L. Pasteur, 1 rue Blessig 67084-Strasbourg Cedex, France |
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| Abstract: | Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic 87Sr from the clays into the surrounding pore waters.The ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays. |
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