Sr, Pb Isotopes and REE Analyses of Five Cores of the Red Sea: An Insight into Hydrothermal Input

Abstract

The formation of the Red Sea was initiated by the separation of the Arabian and Nubian plates and is generally considered to be an embryonic ocean which offers the opportunities to study the transition from a continental to an oceanic rift. Between 16 and 20~ the oceanic crust is continuous, further to the North the axial trough becomes discontinuous and only some isolated deeps contain metalliferous sediments. Hydrothermal fluids flow and could react with the important Miocene evaporite deposits, the recent sediment deposits and the young oceanic crust. The resulting fluids overflow in the deeps and allow the formation of hot, salty, concentrated and reducing brine pools. Sr, Pb isotopic compositions and REE patterns have been determined on sediments and interstitial waters from five cores of the Red Sea sampled during the RedSed cruise in september 1992. Four of these cores were retrived from four deeps located on the axial graben (Atlantis II, Thetis, PortSoudan and Suakin), and one at 10 km from the axial trough. This study yields new constraints on the contribution of the different end-members (biogenic, detritic, basaltic and evaporitic), on the formation of the metalliferous sediments, and on the impact and magnitude of hydrothermalism in these various deeps. The strontium isotopic results of sediment interstitial waters show values similar to those deep water of each site. They are equilibrated with the present brines or with the overlying water for the deep without brines. This argues for water circulation towards the sedimentary column. These waters do not give the Sr isotopic signature corresponding to that present at the time of the deposit. The results obtained on the salt-free sediments trace various sources. Atlantis II deep contains the most metalliferous muds with individualization of sulphide and oxide layers. The sediments of Atlantis II deep present the lowest Sr isotopic composition (0.70702 < 87Sr/86Sr < 0.70817; Fig. 1), a major positive Eu anomaly (Eu/Eu* max = 19.25) and high LREE EOST/CGS CNRS-UMR 7517, 1, rue Blessig, F-67084 Strasbourg cedex, France