Upper crustal rocks (lavas and sheeted dikes) exhibit pervasive alteration due to intense circulation of seawater. Sulfate is the dominant sulfur phase, and sulfur (未34S) and oxygen isotopic compositions of sulfate suggest that sulfate is mainly derived from late Cretaceous seawater. 34S enrichments in sulfides (with respect to mantle sulfur) and low sulfide-S contents (< 10 ppm) reflect precipitation of hydrothermal sulfides followed by oxidation of sulfides during seafloor weathering. Clearly negative 未34S values of CRS (Cr-reducible sulfur) in one pillow basalt sample indicate that microbial reduction of seawater sulfate occurred within the lavas during low-temperature alteration.
Rocks of the gabbro/sheeted dike transition zone display variable 未34SCRS values (between 0.5鈥?and 5.5鈥? paired with a wide range in sulfide-S contents (< 1 to 489 ppm). This reflects the dynamic nature of this interface between the hydrothermal circulation system of the upper crust and the magmatic system dominating the lower crust. Low 未34Ssulfate-S values indicate that sulfate in those rocks mainly results from oxidation of igneous sulfides.
Although lower crustal rocks of the Oman ophiolite are affected by high-temperature alteration resulting in leaching and redistribution of sulfide-S, 未34S values of monosulfide and disulfide minerals scatter around the value for mantle sulfur (0鈥?. 螖33S* values of CRS between 鈭?#xA0;0.020鈥?and 鈭?#xA0;0.038鈥?clearly differ from those of upper crustal rocks. Either this documents leaching processes operating in the lower crust, or lower crustal rocks have preserved a primary multiple sulfur isotopic composition that deviates from the postulated mantle value (螖33S* = 0.0鈥?.
Rocks from the mantle sequence of the Oman ophiolite are affected by serpentinization under highly reducing conditions at low water-rock ratios followed by a further stage of serpentinization under oxidizing conditions at high water-rock ratios. The first stage of alteration results in loss of sulfur (sulfide-S contents < 60 ppm) paired with 未34Ssulfide values around 0鈥? The second stage leads to 34S-enriched sulfide phases and also low sulfide-S contents. Sulfate-S contents vary from 5 ppm to 213 ppm and are generally higher than in the lower crust. Sulfur and oxygen isotopic compositions of sulfate suggest that sulfate is mainly derived from late Cretaceous seawater.