Sub sea floor boiling of Red Sea brines: new indication from noble gas data

Abstract

Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (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 without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henry’s Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in He MORB and Ar MORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.