ORIGIN OF FLUIDS AND THE EVOLUTION OF THE ATLANTIS-II DEEP HYDROTHERMAL SYSTEM, RED-SEA - STRONTIUM ISOTOPE STUDY

Atlantis II is the largest and most mineralized of the deeps along the axis of the Red Sea spreading center. Its basaltic substratum is cove red by recent layered metalliferous sediments, which precipitated from an overlying brine pool. The Sr-87/Sr-86 ratio and the strontium conc entration of interstitial waters within these sediments range between 0.70708 and 0.70725 and between 43 and 53 ppm, respectively. They are close to what is found for the present-day deep brine pool (0.707105, 45.10 ppm). The strontium concentration and the Sr-87/Sr-86 ratio of t he Atlantis II Deep brines can be derived from those of the interstiti al waters of the surrounding Miocene evaporite by hydrothermal interac tion with oceanic basaltic rocks at a maximal water/rock ratio of 2-3. This water/rock ratio is similar to that calculated for oceanic hydro thermal systems on sediment-free ridges. Interstitial waters show a li near trend on a plot of Sr-87/Sr-86 vs. 1/Sr. The highest strontium co ncentration and the most radiogenic interstitial waters correspond to sediment samples enriched in iron and manganese oxide minerals. These waters reflect the diagenetic release of strontium by oxide minerals t hat initially precipitated at the interface between the brine pool and more radiogenic seawater. The solid fraction of the sediment has Sr-8 7/Sr-86 isotopic compositions intermediate to those of the brines and seawater. The most radiogenic strontium values were observed in sample s strongly enriched in marine microbiota. The gradual isotopic evoluti on in the lowest part of the western basin sediments testifies to the gradual influence of the hydrothermal activity in the deep in the begi nning of the Atlantis II Deep sedimentary history. The strontium isoto pic composition of solid samples from younger metalliferous facies is fairly uniform and close to that of the present-day brine. This isotop ic homogeneity indicates that the isotopic composition of mineralizing fluids did not change during the time of deposition of the metallifer ous sediment.