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

Abstract

Brines from three depressions along the axis of the Red Sea, the Atlantis II, the Discovery and the Kebrit Deep, were sampled and analyzed for helium and argon isotopes. We identified two principally different geochemical fingerprints that reflect the geological setting of the deeps. The Atlantis II and the Discovery brines originating from locations in the central Red Sea show 4He concentrations up to 1.2×10−5 cm3 STP g−1 and a 3He/4He ratio of 1.27×10−5. The MORB-like 3He/4He ratio is typical of an active hydrothermal vent system and clearly indicates a mantle origin of the helium component within the brines. 40Ar/36Ar ratios are as high as 305 implying that mantle-derived 40Ar excesses of up to 3% of the total argon concentration are present in the brines and transported along with the mantle helium signal. The mean (4He/40Ar)excess ratio of 2.1 is in agreement with the theoretical mantle production ratio. In the Kebrit Deep, located in the northern Red Sea, we found a helium excess of 5.7×10−7 cm3 STP g−1. The low 3He/4He ratio of 1×10−6 points to a predominantly radiogenic 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 or crustal horizons. In contrast to the Atlantis II and Discovery Deep, the Kebrit brine shows no sign of an active hydrothermal input.