A SULFUR ISOTOPIC PROFILE THROUGH THE TROODOS OPHIOLITE, CYPRUS - PRIMARY COMPOSITION AND THE EFFECTS OF SEAWATER HYDROTHERMAL ALTERATION

The sulfide mineralogy, sulfur contents, and sulfur isotopic compositi ons were determined for a section through the Troodos ophiolite, as re presented by drillcore from Holes CY1A, CY2A, and CY4, plus selected o utcrops, with the goals of understanding the geochemistry of S during hydrothermal alteration of the crust and obtaining a mass balance for sulfur in altered ophiolitic crust. Primary deltaS-34 values of the op hiolite are 0-1parts per thousand, consistent with a MORB- or BAB-like sulfur source. The volcanic rocks have low sulfur contents (mean = 40 ppm) and generally negative deltaS-34 values (to -26.1parts per thous and). These are the results of sulfur loss through oxidation by seawat er at low temperatures (<100-degrees-C) and isotopic fractionation dur ing partial oxidation of igneous sulfides. The sheeted dike complex is uniformly enriched in S-34 (deltaS-34 = 5.4parts per thousand) as the result of mixing of reduced Cretaceous seawater sulfate (17parts per thousand) with primary crustal sulfide. Sulfate was reduced through ox idation of ferrous iron in the dikes and conversion of igneous pyrrhot ite to secondary pyrite. Decreasing water/rock ratios with depth in th e plutonic section led to a general decrease in deltaS-34 values downw ard to igneous values in the lower gabbros. Seawater effects are prese nt locally to the base of the section, however. Cooling of diffuse upw elling hydrothermal fluids (T > 250-degrees-C) produced S and metal en richments in the upper dikes (1.26 wt% S), whereas sulfur was lost fro m the lower dikes and upper plutonic rocks (which contain 200 ppm and 10-1640 ppm S, respectively) through higher temperature (>350-degrees- C) alteration reactions. Epidosites at the dikes-gabbro transition con tain 30 ppm S with deltaS-34 = 5.2-6.5parts per thousand. These rocks lost sulfur and metals during reaction with S-34-enriched deep hydroth ermal fluids at high temperatures (350-400-degrees-C) and high water/r ock ratios in basal hydrothermal upflow zones. The ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur wit hin the crust and exchange of crustal sulfur for seawater sulfur, with little net change in the sulfur content of the crust overall. The bul k altered Troodos crust is enriched in S-34, having deltaS-34 = 3.6par ts per thousand. Fluxes of sulfur between seawater and ophiolitic crus t are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans. The amount of seawater sul fur that reacted with Troodos ophiolite crust is approximately twice t hat for a composite section of oceanic crust, consistent with the gene rally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust.