THE ANHYDRITE SATURATION INDEX OF THE PONDED BRINES AND SEDIMENT POREWATERS OF THE RED-SEA DEEPS

We compiled the available chemical analyses of the ponded brines and o f the sediment interstitial waters for the Atlantis II, Discovery, Sua kin and Valdivia deeps in Be Red Sea central valley, along with the DS DP data for interstitial waters of several cores in the rift axis (Sit es 225-228 of Leg 23). Recent advances in the calculation of the therm odynamic properties of concentrated solutions at high temperatures and pressures allow the study of the equilibrium conditions between anhyd rite and halite, and the brines. We first tested the influence of vari ous parameters as temperature, pressure and solution composition on th e calculated anhydrite saturation index in order to provide a clear cr iterion for equilibrium, which is assumed when the saturation index li es within 0.9 and 1.1. We found that at 62 degrees C and 200 bar, the pressure effect on the saturation indices is of the same magnitude as that of temperature. Our calculations point to the overall undersatura tion of the free brines with respect to halite. The calculated halite saturation state of the pore waters at the four DSDP sites is consiste nt with the mineralogical description of the sediments in which halite is not reported, except at Site 225 where the calculated undersaturat ion is in contradiction with the reported presence of halite at the ba se of the core. From the hypothesis of equilibrium between anhydrite a nd the pore waters in the DSDP cores, we have estimated values of the geothermal gradients off the rift axis which fall within the highest v alues obtained by extrapolation of direct temperature measurements. Th is suggests that, at the latitude of the Atlantis II and Suakin deeps, the high heat fluxes measured along the axial trough extend off the r ift axis. Our calculations show that the Upper Convective Layer (UCL) of the Atlantis II hydrothermal system has always been undersaturated over the studied period (1965-1985), which is consistent with the abse nce of anhydrite in sediment underlying this brine. The calcium and su lfate contents of the Lower Convective Layer (LCL) show a parallel evo lution: they decrease when temperature increases. Although this behavi our suggests control of brine chemistry by equilibrium with anhydrite, our calculations show that anhydrite has reached saturation in the lo wer brine only in 1966 and 1976. The general regime leading to the los s of calcium and sulfate outside these periods is tentatively attribut ed to diffusion. Calculations show that, throughout the whole stratifi ed brine column (from the sediment to normal Red Sea water),there is n o chemical potential gradient for calcium sulfate whereas the chemical potential of sodium chloride regularly decreases. We tentatively advo cate the coupling of calcium sulfate diffusion with the large sodium c hloride gradient to account for the loss in Ca and SO4. Finally, the p ore water chemistry of the Atlantis II and Discovery Deep sediments re cord two parallel evolutions from anhydrite undersaturation in 1966 to saturation (or a state close to) in 1976. These data are consistent w ith a periodical brine overspill from Atlantis II into Discovery.