GEOLOGY AND GEOCHEMISTRY OF SEEP CARBONATES FROM BUSH HILL OF THE GULF OF MEXICO

Seven seep carbonates from Bush Hill (GC185) seep and hydrate site of the Gulf of Mexico are studied by sedimentary petrology and geochemistry. The result is that the seep carbonates occur as crusts. The crusts mainly consist of microcrystalline, microspar, and sparite aragonites (89%~99% with an average of 94%). Seep community, e.g. bivalve shells and tube worms are well preserved in the crusts. The moderate ¹³C depletion of the carbonates (the most depleted one has δ¹³C value of -29.4‰) indicates that non-methane hydrocarbons (C₅⁺) were incorporated during the precipitation of the carbonates. Relatively enrichment of ¹⁸O in the carbonate (δ¹⁸O:+2.4‰~+5.0‰ PDB) may be related to in situ destabilization of gas hydrate. The total content of rare earth element (REE) of the 5% HNO₃^- treated solution of the carbonates is low (0.40~20.85×10^-6). The shale-normalized REE patterns show varied Ce anomalies(significantly negative, slightly negative, and no Ce anomalies). Ce anomalies are obvious in different samples and even in the same sample of different degrees of crystallization(microcrystalline, microspar and sparite),suggesting that the formation conditions of the Bush Hill seep carbonate is variable and complex, which is possibly controlled by the rate of fluids flow. We investigated the heterogeneity of isotopes and trace elemental concentrations on micro-scales in a seep site of Bush Hill of the Gulf of Mexico and its importance would be providing an example for studying the cold seeps in other regions and in ancient record.