美国俄勒冈东部晚三叠世冷泉碳酸盐岩流体来源及沉积环境

Fluid sources and formation environment of Late Triassic seep carbonate in eastern Oregon,United States

冷泉碳酸盐岩作为记录冷泉系统中烃类厌氧氧化过程的重要载体,其岩石学和地球化学特征能够很好地反映冷泉流体来源及沉积氧化还原环境信息。美国俄勒冈地区发育大量晚三叠世冷泉碳酸盐岩,为探究其成岩流体性质并重建沉积环境,本文对其进行了详细的岩石学、碳氧同位素、微量和稀土元素含量分析。结果表明,该碳酸盐岩主要由泥微晶方解石组成,保存了原始矿物——文石的针状和环带状结构,指示其形成于高硫酸根浓度孔隙水中;其δ^(13)C为–32.7‰~–5.1‰,镜下还可见大量原生沥青,表明碳酸盐岩的碳源为甲烷、原油和海水;其稀土元素组成特征显示轻稀土亏损、重稀土富集,Ce呈明显负异常,Y/Ho平均值为45.30,与海水配分模式相似。综合研究表明,俄勒冈冷泉碳酸盐岩形成于靠近海底的浅表层氧化沉积环境中。

As an important carrier of the anaerobic oxidation process of hydrocarbons in seep systems,the petrological and geochemical characteristics of seep carbonate play a crucial role in tracing the source of fluids and the redox environment.A large number of Late Triassic seep carbonates were developed in the Oregon region of the United States.In order to investigate the characteristics of the diagenetic fluids and to reconstruct the sedimental redox environment,comprehensive petrological,carbon and oxygen isotope,trace and rare earth element content analyses were carried out in this paper.Our findings reveal that these carbonates are predominantly composed of microcrystalline calcite,preserving the needle and banded structure of aragonite,indicating that it was formed in pore water with high sulfate concentration.The carbon isotopic values(δ^(l3)C)range from-32.7‰ to-5.1‰,and a large amount of native bitumen is also visible under the microscope,suggesting that the carbon sources of the carbonate were methane,crude oil,and seawater.Rare earth elements(REEs)exhibit characteristics of light REEs depletion and heavy REEs enrichment,with a pronounced negative anomaly in Ce and an average Y/Ho ratio of 45.30,which is similar to the seawater partitioning pattern.In summary,above results indicate that the Oregon seep carbonates were formed in a shallow oxidized sedimentary environment close to the seafloor.