A profile of multiple sulfur isotopes through the Oman ophiolite

Abstract

The mineralogy, sulfur contents, and multiple sulfur isotopic compositions were determined for a complete section through the Oman ophiolite at the Wadi Gideah area located in the Wadi Tayin Massif (southern domain of the ophiolite complex) in order to characterize the various processes of sulfur cycling in the Oman ophiolite which is regarded as representing the best example of fast-spreading oceanic lithosphere on land.Upper crustal rocks (lavas and sheeted dikes) exhibit pervasive alteration due to intense circulation of seawater. Sulfate is the dominant sulfur phase, and sulfur (δ34S) 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 (<10ppm) reflect precipitation of hydrothermal sulfides followed by oxidation of sulfides during seafloor weathering. Clearly negative δ34S 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 δ34SCRS values (between 0.5‰ and 5.5‰) paired with a wide range in sulfide-S contents (<1 to 489ppm). 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 δ34Ssulfate-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, δ34S values of monosulfide and disulfide minerals scatter around the value for mantle sulfur (0‰). Δ33S* values of CRS between −0.020‰ and −0.038‰ 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 (Δ33S*=0.0‰).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 <60ppm) paired with δ34Ssulfide values around 0‰. The second stage leads to 34S-enriched sulfide phases and also low sulfide-S contents. Sulfate-S contents vary from 5ppm to 213ppm 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.