Sulfur Isotope Reconnaissance of Epigenetic Pyrite in Ocean-floor Basalts, Leg 34 and Elsewhere

Abstract

Sulfur isotope analyses of 10 epigenetic occurrences of pyrite in ocean-floor basalts yield an unexpectedly large range of á 34 S values (from -24.2 to +23.O°/oo) for sulfides that are inferred to be of hydrothermal origin. The existing geologic, mineralogical, and isotopic data do not permit an unambiguous interpretation of the chemical and thermal characteristics of these hydrothermal systems. They fit either of two hypothetical models that assume isotopic equilibration between varying proportions of oxidized and reduced species of sulfur, but with pyrite deposition either (1) at 150°C and less from an infinite reservoir of heavy (+21°/oo) seawater sulfur, or (2) at 300°C and less from a finite reservoir of deep-seated (O°/oo) "magmatic" sulfur. Further detail and elaboration of these or alternative models must await additional mineralogical and isotopic studies, particularly of coexisting hydrothermal assemblages.

Sulfur and Carbon Isotope Ratios of Dacite and Rhyolite Conglomerates from Site 439, Leg 57, Deep Sea Drilling Project

“…In quite a few samples of ocean floor basalts from the East Pacific Rise, epigenetic pyrites as heavy as + 24 per mill have been found (Field et al, 1976). Field et al (1976) proposed that magmatic sulfur could have been fractionated into such heavy sulfide through partial oxidation of sulfide to sulfate and sulfur isotope fractionation between them.…”

“…Field et al (1976) proposed that magmatic sulfur could have been fractionated into such heavy sulfide through partial oxidation of sulfide to sulfate and sulfur isotope fractionation between them. Such a reaction should also produce larger amounts of sulfide, which is closer to or even (Sakai et al, 1978).…”

“…The δ 34 S values of the island arc volcanic rocks are about 0 to + 5 per mill (Ueda and Sakai, unpublished data), whereas those of fresh basalts from the ocean floor (Kanehira et al, 1973;Grinenko et al, 1975;Sakai et al, 1978), from Hawaii, (Ueda, and Sakai, unpublished data), and from Iceland (Sakai, Ueda, and Tomasson, unpublished data) range from -0.5 to +1 per mill. In quite a few samples of ocean floor basalts, however, the stable isotope ratios of sulfur, especially those of epigenetic pyrite, show a significant deviation from the normal values (Field et al, 1976;Krouse et al, 1977;Sakai et al, 1978). Those authors have posited various mechanisms of isotope fractionation to interpret the unusual values.…”

Secondary Minerals of Deuteric and Diagenetic Origin Filling Voids and Encrusting Surfaces on Basalts from Deep Sea Drilling Project Leg 54

“…From sulfur isotope studies, Field et al (1976) gave two explanations for the presence of sulfur in pyrite of assumed hydrothermal origin from ocean-floor basalts. They were that the sulfur came from either a finite reservoir of deep-seated magmatic sulfur or from an infinite reservoir of heavy sea-water sulfur.…”

Sulfur Isotope Ratios of Leg 126 Igneous Rocks

“…The sulfate must arise from a process that yields little or no fractionation at all, such as oxidation. However, such oxidation cannot involve the formation of thiosulfate ions (Field et al, 1976), because that would cause a large isotope fractionation.…”