南海北部烟囱状碳酸盐岩记录的冷泉流体活动演化特征研究

Variations in fluid sources and seepages archived in carbonate chimneys from the northern South China Sea

冷泉碳酸盐岩是冷泉渗漏活动的重要产物之一,它记录了冷泉流体的来源、其活动演化特征和沉积环境等。通过对南海北部东沙海域Site3站位采集的三个烟囱状冷泉碳酸盐岩的矿物成分、碳氧同位素及碳酸盐岩中黄铁矿硫同位素的研究,确定了冷泉渗漏可能的流体来源,结合烟囱状碳酸盐岩独特的形貌特征,探讨了研究区冷泉流体的活动演化特征。烟囱A为环状,外环直径16 cm,中央流体通道直径5 cm;烟囱B呈底部直径8 cm的实心锥形;烟囱C也为环状,外环直径20 cm,中央流体通道直径12 cm。三个烟囱都具有较负的碳同位素组成(δ^(13)CVPDB值低至–55.7‰)和较正的氧同位素组成(δ^(18)OVPDB值高达5.4‰)特征。研究认为,甲烷是该海域冷泉渗漏流体的主要烃类来源,其中沉淀烟囱C的冷泉流体中很可能存在水合物分解释放流体的加入。三个烟囱状碳酸盐岩的外层相比于内层,普遍具有δ^(13)C值高、δ^(18)O值低、δ^(34)S值高的特征,认为这是由于烟囱状碳酸盐岩从外至内形成过程中沉积空间逐渐受到限制导致流体渗漏强度增强而造成的。综上所述,烟囱状冷泉碳酸盐岩因其独特的形态结构,是冷泉流体渗漏特征在时间和空间上变化的优良记录者。

Seep carbonates represent excellent archives of fluid sources and dynamics of seepage activities near the seafloor. Three carbonate chimneys recovered from Site 3 of the northern South China Sea were studied using mineralogical and geochemical analyses, including those of carbonate carbon and oxygen isotopes, sulfur isotopes of pyrite, and maj or element composition of the bulk samples. Chimney A is a pipe with an outer diameter of 16 cm and a central fluid passage diameter of 5 cm; chimney B is a cone with a bottom diameter of 8 cm; and chimney C is also a pipe with an outer diameter of 20 cm and a central fluid passage diameter of 12 cm. These chimneys all share a negative carbon isotope composition （ δ13CVPDB values as low as -55.7‰） and positive oxygen isotope composition （δ18 O VFDB values as high as 5.4‰）. Methane represents the predominant carbon source of all carbonates. Incorporation of 18O-enriched fluids from destabilization of gas hydrates is also suggested （chimney C）. In addition, we report that the outer layer of each chimney generally bears higher δ13C values, lower δ18O values, and higher δ34S values than the inner layers of the chimneys. These variations are presumed to reflect a gradual increase in the fluid seepage intensities during the formation of the chimneys, probably due to the limitation of the formation space from the outer layer to the inner layer of the chimneys. In summary, variations in the sources of fluids and seepage dynamics can be archived in carbonate chimneys.