LIQUID HYDROCARBON-BEARING INCLUSIONS IN MODERN HYDROTHERMAL CHIMNEYS AND MOUNDS FROM THE SOUTHERN TROUGH OF GUAYMAS BASIN, GULF OF CALIFORNIA

Liquid hydrocarbon-bearing inclusions of variable shape (e.g. spherical, bowling pin, rod, and highly irregular) occur in hydothermal minerals of chimneys and mounds in the southern trough of Guaymas Basin, central Gulf of California. The inclusions are preferentially trapped in amorphous silica which occurs as spherules and 0.01 mm-thick encrustations on sulfides, sulfates and carbonates. The inclusions are primary and pseudosecondary and were trapped during, not after, mineral growth. The inclusions are both two-phase (liquid hydrocarbon and vapor/gas) and three-phase (liquid hydrocarbon, aqueous fluid and vapor), and range from 2 to 50 μm in diameter. The large range in liquid hydrocarbon and acqueous fluid contents indicates that the hydrothermall fluid and liquid hydrocarbon were never a homogeneous solution, but that hydrocarbons were transported as immiscible and, possibly, solvated forms. The liquid hydrocarbon varies in color from deep to pale orange-brown and fluoresces a yellow color during excitation by u.v. light, indicating a condesate (mainly C1 to ∼C10 hydrocarbons) composition. Measurable quantities of hydrocarbons from the inclusions could not be isolated by mineral dissolution, by pyrolsis and decrepitation analyses suggest that the more volatile components predominate. Two-phase hydrocarbon inclusions homogenized with the liquid at temperatures ranging from 75 to 190°C (excluding one high measurement). To obtain true trapping temperatures, an unknown pressure correction is required; however, the trapping temperature of these hydrocarbon inclusions was estimated by measuring trapping temperatures for adjacent aqueous inclusions which range from 116 to 226°C. Geochemical modelling using these trapping temperatures indicates that the amorphous silica precipitated from the hydrothermal fluid due to conductive cooling in addition to mixing with ambient sea water. The temperatures required for the formation of petroleum and its transport to the seafloor in the Guaymas Basin coincide with those necessary for the precipitation of amorphous silica in the Guaymas Basin hydrothermal system. © 1990.