Sub sea floor boiling of Red Sea Brines: New indication from noble gas data

Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled i n situ and analyzed for noble gases. The atmospheric noble gas concentratio ns (Ne, Ar-atm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30 % relative to the initial concentrations in ambient Red Sea Deep Water with out a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase se paration is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical c onditions, in which gases are partitioned according to Henry's Law, we reco nstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it bec omes enriched in He-MORB and Ar-MORB, the ascending fluid boils, and the re sidual liquid becomes depleted in noble gas concentrations. The depleted fl uid rises to the sediment surface and feeds the Atlantis II basin. The rela tively low boiling degree of about 38 (i.e., the percentage of fluid remove d as vapor) derived from the model indicates that the Atlantis II system re presents an early stage of boiling with relatively small gas loss, in contr ast to hydrothermal systems at sediment-free mid-ocean ridges. Copyright (C ) 2000 Elsevier Science Ltd.