Chemical and isotopic (87Sr/86Sr, δ18O, δD) constraints to the formation processes of Red-Sea brines

About twenty deeps filled with hut brines and/or metalliferous sediments, are located along the Red-Sea axis. These brines present a well-suited framework to study the hydrothermal activity in such a young ocean. The present study outlines the results of a geochemical approach combining major-, trace-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 temperature. 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 unique formational model for all of the brines. For example, the brines of the Suakin deep appear to have been formed by an old sea water which dissolved evaporite beds, without significant fluid circulation and hydrothermal input, while others such as Atlantis II or Nereus Deeps appear to be dominated by hydrothermal influences. A striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity. Copyright (C) 2001 Elsevier Science Ltd.