A sulfur isotopic profile through the troodos ophiolite, Cyprus: Primary composition and the effects of seawater hydrothermal alteration

Abstract

The sulfide mineralogy, sulfur contents, and sulfur isotopic compositions were determined for a section through the Troodos ophiolite, as represented by drillcore from Holes CY1A, CY2A, and CY4, plus selected outcrops, 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 δ34S values of the ophiolite are 0–1‰, consistent with a MORB- or BAB-like sulfur source. The volcanic rocks have low sulfur contents (mean = 40 ppm) and generally negative δ34S values (to −26.1‰). These are the results of sulfur loss through oxidation by seawater at low temperatures (<100°C) and isotopic fractionation during partial oxidation of igneous Sulfides. The sheeted dike complex is uniformly enriched in 34S (δ34S = 5.4‰) as the result of mixing of reduced Cretaceous seawater sulfate (17‰) with primary crustal sulfide. Sulfate was reduced through oxidation of ferrous iron in the dikes and conversion of igneous pyrrhotite to secondary pyrite. Decreasing water/ rock ratios with depth in the plutonic section led to a general decrease in δ34S values downward to igneous values in the lower gabbros. Seawater effects are present locally to the base of the section, however. Cooling of diffuse upwelling hydrothermal fluids (T > 250°C) produced S and metal enrichments in the upper dikes (1.26 wt% S), whereas sulfur was lost from the lower dikes and upper plutonic rocks (which contain 200 ppm and 10–1640 ppm S, respectively) through higher temperature (>350°C) alteration reactions. Epidosites at the dikes-gabbro transition contain 30 ppm S with δ34S = 5.2–6.5‰. These rocks lost sulfur and metals during reaction with 34S-enriched deep hydrothermal fluids at high temperatures (350–400°C) and high water/rock ratios in basal hydrothermal upflow zones. The ultimate effect of alteration of ophiolitic crust is redistribution of igneous sulfur within the crust and exchange of crustal sulfur for seawater sulfur, with little net change in the sulfur content of the crust overall. The bulk altered Troodos crust is enriched in 34S, having δ34S = 3.6‰. Fluxes of sulfur between seawater and ophiolitic crust are an order of magnitude smaller than modern riverine input to and sedimentary sulfide output from the oceans. The amount of seawater sulfur that reacted with Troodos ophiolite crust is approximately twice that for a composite section of oceanic crust, consistent with the generally greater intensity of alteration of ophiolitic crust compared to modern oceanic crust.