Mineralogical and geochemical characterization of hydrocarbon seep sediments from the Gulf of Mexico

摘要

A common characteristic of the Gulf of Mexico (GOM) seafloor,hydrocarbon seepage produces asphalt volcanism,brine pools,gas hydrates and authigenic carbonates.We studied authigenic carbonates from two areas:Green Canyon (Louisiana slope,1000 mbsl) and the Chapopote diapir (abyssal plain,southwestern GOM,2900 mbsl).Carbonates consist of oily,matrix-supported limestones with variable porosity and a fine-grained matrix of calcite.They occur in Green Canyon as both concretions formed below the seafloor and as slabs of seabed pavements.In each case,the carbonates are mudstones with a mud matrix consisting of high-Mg calcite,disseminated pyrite and detrital quartz.The carbonate slabs have vug microporosity and contain barite that occurs as vug lining.The precipitation of barite results from a reaction between barium-rich and sulfate-free seep fluids and downwards-diffusing seawater.The carbonates in Chapopote occur in relation to tar flows and form crusts of wackestones with bivalve shells.The matrix is composed by low-Mg calcite with disseminated pyrite and detrital quartz.The carbonate mud shows depletion in ~(13)C,with delta~(13)C between -36.9 per thousand and -23.0 per thousand.Carbonates precipitate as a consequence of anaerobic methane oxidation coupled with seawater sulfate reduction.delta~(18) values (3.4-44 per thousand) suggest precipitation at temperatures between 5 and 9 deg C.