Reduced Fluids and Sulfide–Metal Alloy in the Magnesian Basalts of Disko Island, West Greenland

Abstract

This paper presents the results of a combined study of the composition and physical properties of fluid, melt and opaque mineral inclusions in olivine phenocrysts and glass from the magnesian basalt of Disko I., West Greenland. The rock is dominated by glass, which contains numerous fluid bubbles and large (up to 250 μm) opaque globules. The thermometric investigation of the melt inclusions showed that the temperature of olivine crystallization was no higher than 1220°C at a pressure of 0.15–0.20 GPa. The opaque globules in the groundmass glass have a eutectoid structure and are composed of troilite, Fe–Ni metal alloy, and sparse grains of wüstite and cohenite (Fe₃C). Cryometric measurements and Raman spectroscopy indicated a complex fluid composition and presence of reduced and oxidized gases: CH₄, N₂, H₂, CO₂, and H₂O, as well as highly ordered graphite. The nonequilibrium association of compounds in the fluid is related to the rapid cooling (quenching) of crystallizing magma preventing the equilibration of the gas system. At an estimated logfO₂ value of –13.95, methane is the only hydrocarbon phase that can exist at magmatic temperatures. The formation of organic substances detected in some gas bubbles in the groundmass glasses of the rock occurred at postmagmatic stages after a significant decrease in temperature.