Origin of sulphur and geothermometry of hydrothermal sulphides from the Galapagos Rift, 86 °W

Abstract

Polymetallic hydrothermal massive sulphides from a mid-ocean ridge were first recovered in 1978^1,2, from the East Pacific Rise (EPR) at 21° N, and later ‘black smokers’ were found³ debouching fluids at 380°±30 °C. The Galapagos Rift and Fracture Zone Intersection (GRAFZI) Project of 1980 discovered sulphides in inactive mounds with up to 27% Cu and 31% Fe at 86° W on the Galapagos Rift⁴, that are chemically very different from the EPR 21° N sulphides (up to 50% Zn). Stable isotope studies were initiated in an attempt to find the origin of the sulphide sulphur, to determine the temperature of formation of an equilibrium assemblage of chalcopyrite (CuFeS₂) and pyrite (FeS₂), and to understand the operative seawater/rock ratio, and therefore the controls on permeability of the oceanic crust responsible for producing hydrothermal sulphides. There has been relatively little similar work on samples from 21° N (refs 5–7). We report here results which indicate sulphide formation at 390°±20 °C that is almost identical to that of the 21 °N black smokers which were measured directly by temperature probe. The seawater sulphate contribution to the sulphur of the sulphides was much more dominant than at 21° N (ref. 7), and suggests a more fractured crust and/or more rapid hydrothermal solution flow rates, induced from higher seawater/rock ratios.