GEOLOGY AND GEOCHEMISTRY OF SEEP CARBONATES FROM BUSH HILL OF THE GULF OF MEXICO
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摘要: 对墨西哥湾布什山采集的7块冷泉碳酸盐岩样品进行了沉积岩石学及地球化学研究,结果表明:冷泉碳酸盐岩以结壳的形式产出,结壳中双壳类和管状蠕虫等冷泉生物碎屑发育,结壳主要由泥晶、微晶和亮晶文石组成(含量89%~99%,平均94%)。碳酸盐岩13C中等亏损(δ13C最负值为-29.4‰,PDB),表明碳的来源主要是非甲烷的碳烃化合物,18O相对富集(δ18O为+2.4‰~+5.0‰,PDB),可能与原地水合物分解形成富集18O的孔隙水有关。碳酸盐矿物相(5%硝酸可溶部分)的稀土元素总量较低(0.40×10-6~20.85×10-6),稀土元素Ce异常特征和页岩标准化配分模式在不同样品、甚至同一样品结晶程度不同的碳酸盐矿物相(泥晶、微晶和亮晶)中变化显著,表明布什山冷泉碳酸盐岩形成过程中氧化还原条件复杂多变,控制氧化环境变化的主要因素可能是流体的渗漏速率。Abstract: 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.
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Key words:
- seep carbonate /
- stable carbon and oxygen isotopes /
- rare earth elements /
- Bush Hill /
- Gulf of Mexico
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