Anhydrite sulfur isotopes

Kusakabe, M. and Chiba, H. (1983) Oxygen and sulfur isotopic composition of barite and anhydrite Irom the Fukazawa deposit, Japan. Econ. Geol. Mon. 5, 292-301. 

Fig. 2.43. Graphical illustration of sulfur isotope values of HiS (left axis and. solid line) produced during basalt-seawater interaction at various water/rock ratios. Calculations assume that seawater sulfate is mostly removed as anhydrite, that any residual sulfate is reduced by iron oxidation in reacting basalt, and that there is quantitative leaching of basaltic sulfide and homogeneous mixing of both sulfides. Dashed line...

As pointed out by Seal et al. (2000), many studies of ancient hydrothermal systems have utilized equilibrium sulfate-sulfide sulfur isotope fractionation models, but these should be applied with great caution. As shown in Figure 9, seafloor hydrothermal vent fluid 5" Sh2S values do not conform to simple equilibrium fractionation models. Shanks et al. (1981) first showed experimentally that sulfate in seawater-basalt systems is quantitatively reduced at temperatures above 250°C when ferrous minerals like the fayalitic olivine are present. When magnetite is the only ferrous iron-bearing mineral in the system, sulfate-reduction proceeds to sulfate-sulfide equilibrium, but natural basalts contain ferrous iron-bearing olivine, pyroxene, titanomagnetite, and iron-monosulfide solid-solution (mss) (approximately pyrrhotite). It is the anhydrite precipitation step... 

In all samples studied, of pyrite is the most depleted. The OM sulfur is on average 10%c heavier a very small amount of the sulfate minerals gypsum and anhydrite for the Nabi Musa location indicates evaporative conditions and sulfate closed system. Despite the same Senonian Ghareb Formation sedimentary rock for aU of the Dead Sea area samples studied, the values recorded for different sites show wide variability. This indicates that small differences in sub-basin conditions have a significant imprint on sulfur isotope distribution. 

---