HYDROTHERMAL ACTIVITY AS RECORDED BY RED-SEA SEDIMENTS - SR-ND ISOTOPES AND REE SIGNATURES

Three cores from different Red Sea deeps were analysed for Sr-Nd isotopes and REE. Non-metalliferous biodetrital sediment from the Shaban Deep was used as reference. Its REE-pattern characteristics are similar to those of normal detrital sediment, whereas the Sr and Nd isotope ratios (0.7092 and epsilon(Nd) = -4.7) are consistent with deposition from seawater. Sediment of the most metal-enriched hydrothermal core from the Atlantis II Deep shows HREE-depleted patterns with a large positive Eu anomaly, typical of high-temperature fluids. Sr and Nd isotope compositions are the same throughout the core (0.7075 and epsilon(Nd) = -0.9) and very different from those of the Shaban Deep. The third core, also from the Atlantis II Deep, consists of biodetrital sediment at the base and of increasingly metalliferous sediment toward the top. The positive Eu anomaly becomes larger as Sr-87/Sr-86 decreases upward reaching a maximum Eu/Eu* value at 2.58 and a minimum Sr-87/Sr-86 at 0.7076. Sediment containing fish remains (Oudin and Cocherie, 1988) has a high SIGMAREE (776 ppm), its high epsilon(Nd) (+4.0) indicating that the fluid interacted with underlying basalt. To construct the curve of Nd-evolution in seawater with time, use of the Nd isotope composition of sediment must be accompanied by REE determinations, in order to select samples without potential hydrothermal fluid interaction, as the latter can increase the isotope ratio of Nd.