Isotopic composition of pyritic sulfur from the mud volcanic ejecta in Azerbaijan

Geochemical studies of pyrite crystals from the mud volcanic ejecta in Azerbaijan were studied. It is shown that all of them have cubic shape. Determination of the sulfur isotope composition revealed a wide variation range of δ³⁴S values from–27.0 to +26.4‰. Signs of spatial zonation were recorded in the distribution of δ³⁴S values—lower values are confined to the present-day coastline of the Caspian Sea. Appearance of pyrite with a high share of ³⁴S is attributed to sulfate reduction that takes place in an environment with excess organic matter. It is supposed that the isotopically heavy sulfides represent the “neck” facies that are formed at the periphery of mud volcanic conduits at the contact of the hydrocarbon-rich mud volcanic fluids with stratal waters of host sediments.