Constraints on origin and evolution of Red Sea brines from helium and argon isotopes

Brines from three depressions along the axis of the Red Sea, the Atlantis I I, the Discovery and the Kebrit Deep, were sampled and analyzed for helium and argon isotopes. We identified two principally different geochemical fin gerprints that reflect the geological setting of the deeps. The Atlantis II and the Discovery brines originating from locations in the central Red Sea show He-4 concentrations up to 1.2 X 10(-5) cm(3) STP g(-1) and a He-3/He- 4 ratio of 1.27 x 10(-5). The MORE-like 3He/4He ratio is typical of an acti ve hydrothermal vent system and clearly indicates a mantle origin of the he lium component within the brines. Ar-40/Ar-36 ratios are as high as 305 imp lying that mantle-derived Ar-40 excesses Of up to 3% of the total argon con centration are present in the brines and transported along with the mantle helium signal. The mean (He-4/Ar-40)(excess) ratio of 2.1 is in agreement w ith the theoretical mantle production ratio. In the Kebrit Deep, located in the northern Red Sea, we found a helium excess of 5.7 x 10(-7) cm(3) STP g (-1). The low He-3/He-4 ratio of 1 x 10(-6) points to a predominantly radio genic source of the helium excess with only a minor mantle contribution of approximately 9%. We propose a new scenario assuming that the Kebrit brine accumulates a diffusive helium flux that migrates from deeper sedimentary o r crustal horizons. In contrast to the Atlantis II and Discovery Deep, the Kebrit brine shows no sign of an active hydrothermal input. (C) 2001 Elsevi er Science B.V. All rights reserved.