Chemical and isotopic (Sr-87/Sr-86, delta O-18, delta D) constraints to the formation processes of Red-Sea brines

About twenty deeps filled with hut brines and/or metalliferous sediments, a re located along the Red-Sea axis. These brines present a well-suited frame work to study the hydrothermal activity in such a young ocean. The present study outlines the results of a geochemical approach combining major-, trac e-element and isotopic (oxygen, hydrogen, strontium) analyses of brines in six of the deeps, to evaluate different processes of brine formation and to compare the evolution of each deep. Important heterogeneities in temperatu re. salinity, hydrographic structure and chemistry are recorded, each brine having its own characteristics. The intensity of hydrothermal circulation varies among the deeps and ranges from being strong (Atlantis II and Nereus ) to weak (Port-Soudan) and even to negligible (Valdivia and Suakin) and it varies along the entire Red-Sea axis. These observations do not favour a u nique formational model for all of the brines. For example, the brines of t he Suakin deep appear to have been formed by an old sea water which dissolv ed evaporite beds, without significant fluid circulation and hydrothermal i nput, while others such as Atlantis II or Nereus Deeps appear to be dominat ed by hydrothermal influences. A striking feature is the absence of a relat ionship between the position of the deeps along the axis and their evolutio nary maturity. Copyright (C) 2001 Elsevier Science Ltd.