Michel Hoffert

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays

Abstract Morphological, mineralogical, chemical and RbSr 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 87 Sr 86 Sr 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 87 Sr 86 Sr 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 87 Sr 86 Sr 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 87 Sr 86 Sr 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.