Sulphur isotope analyses of 62 samples from the Shetland ophiolite, Scotland, show that the sulphides throughout the igneous stratigraphy have a magmatic signature with the exception of the uppermost gabbros and the lowermost basal thrust talc-carbonate lithologies. Mean δ 34S values for whole-rock samples from harzburgite (+3.5‰), dunite pods in harzburgite (+4.3‰), crustal dunite and wehrlite units (+2.5‰), the main pyroxenite unit (+2.9‰), the high-level pyroxenite (+4.1‰) and gabbro (+4.3‰) suggest that the primary sulphides derived their sulphur from a magmatic source enriched in 34S, which is compatible with the supra-subduction zone origin proposed for the Shetland ophiolite. δ 34S values for pyrite mineralization in a gabbro, amphibole pegmatite and plagiogranite from the uppermost part of the gabbros range from +9.6 to +14.0‰ and are thought to result from precipitation from a 34S-enriched fluid, which, by analogy with other studies of ophiolitic crust, could have been derived from sea-water sulphate. A sulphide-bearing dunite from a dunite pod in mantle harzburgite at the Cliff locality that contains 4 ppm ∑Pt + Pd has an average δ 34S value of +4.2‰, indistinguishable from values from other non-mineralized dunites from dunite pods and indicating a magmatic origin for this sulphur. In contrast, talc-carbonate altered rocks from the basal thrust, including samples from close to the Cliff locality, have an anomalously low mean δ 34S value of +0.8‰, which can be explained by introduction of a proportion of isotopically light sulphur from the underlying metasedimentary sequence, which has a mean δ 34S of -4.2‰. Stepped combustion analysis supports mineralogical observations that the majority of the sulphur is contained in disseminated Ni, Fe and Cu sulphides. δ 34S laser probe data for intergrown pentlandite and heazlewoodite from the crustal dunite unit are essentially identical, which supports the idea that the replacement of pentlandite by heazlewoodite during serpentinization is isochemical with respect to sulphur.
|Number of pages
|Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science
|Published - 30 Sept 1997
- sulphur isotope
- Shetland ophiolite complex