南海东北陆坡烟囱状冷泉碳酸盐岩生长剖面的碳、氧同位素特征与生长模式

 对SO177航次采集自南海东沙东北古冷泉活动区的烟囱状碳酸盐岩样品（TVG14 C1 1）进行解剖研究，探讨烟囱样品的形成机理。我们首先对烟囱样品的横截面进行高分辨率精细取样并进行碳氧同位素分析，在充分了解其矿物组成特征的前提下，利用碳酸盐岩-水体系氧同位素分馏方程计算古冷泉流体的氧同位素组成并定量分析流体的端元成分和相对贡献，然后根据它们在烟囱生长剖面上的变化特征，为碳酸盐质烟囱建立生长模式。分析表明，该样品横截面的δ13C值在-50.136‰～-43.923‰之间变化，δ18O值在2.762‰～4.848‰之间变化，由中心向外，碳氧同位素呈反向协同变化趋势，δ13C 逐渐升高而δ18O逐步降低。结合该样品的年龄和古海水的氧同位素组成，计算得到形成该样品的冷泉流体的δ18O在1.2‰~2.3‰ V SMOW之间变化，较冰期海水的更富18O。认为在形成烟囱的不同阶段，水合物分解产生的富18O流体与同期海水发生了不同程度的混合，烟囱中心部位水合物分解水的相对贡献高达53.6％，而烟囱外层，水合物分解水的贡献低至6.1‰。通过综合研究，提出了烟囱样品的生长模式。

Carbon and Oxygen Isotope Characteristics of the Growth Profile of a Seep Carbonate Chimney from the Northeastern Slope of the South China Sea and Its Formation Model

This chimney(TVG14 C1 1) was collected by SO177 cruise in 2004 from the northeastern Dongsha, South China Sea. It is composed of Mg calcite.  Author did high resolution sampling on a cross section of the chimney to analyze their C and O isotope compositions. Based on their mineral composition, we used carbonate water fractionation equation to calculate the δ18O of the precipitating fluid and discussed the fluid sources. We showed that the δ13C of the chimney varied from -50.14‰ to -43.92‰, and δ18O from 2.76‰ to 4.85‰. From the inner to the outer, the δ13C increased gradually while δ18O was decreaseing. The equilibrium δ18O of the precipitating fluid were in the range of 1.2‰-2.3‰V SMOW, which were significantly heavier than the modern seawater or glacial water. It is suggested that the water released from gas hydrate dissociation were responsible for the 18O enrichment. According to the two end-member mixing model, it was calculated that the precipitating fluid consisted of 53.6% gas hydrate water and 46.4% seawater during the precipitation stage of the inner layer. It got diluted gradually when it diffused into the sediments to form the outer layer, where the contribution of gas hydrate water decreased to 6.1%. Based on the growth structure and the variation of carbon and oxygen isotope composition, a growth model of the seep carbonate chimney was proposed.