南海北部烟囱状冷泉碳酸盐岩的沉积环境分析

摘要

对南海北部陆坡具有圈层结构的烟囱状冷泉碳酸盐岩的岩石学、矿物学和地球化学特征进行了研究,结果表明:烟囱内外层具有明显不同的特征,内层碳酸盐含量比外层高,内层的碳酸盐矿物也更富含MgCO3,并且矿物颗粒较大,结晶程度较高；外层的铝、硅等陆源碎屑元素比内层含量高.内外层的碳、氧同位素也具有明显差别.对于研究的两个样品,内层的δ13 C分别为-40.80×10-3和-31.27×10-3,δ18 O分别为2.67×10-3和2.00×10-3,而外层的δ13 C分别为-31.26×10-3和-30.99×10-3,δ18O分别为0.48×10-3和1.85×10-3.结合前人在该区域附近的研究结果认为,烟囱状碳酸盐岩具有内层比外层相对较负δ13C和较为富集δ18O的特征.1 mm间隔取样的δ13C和δ18O剖面变化与烟囱状碳酸盐岩的圈层结构吻合较好,由外层至内层δ13C的变化范围为-27.50×10-3～-32.05×10-3,δ18O的变化范围为0.78×10-3～2.34×10-3.外层6 mm的δ13 C和δ18O具有明显的负线性相关,相关系数γ2为0.996 7.根据上述特征,讨论了渗漏甲烷流体的性质、扩散机制及对沉积环境的影响,推测具有圈层结构的烟囱状碳酸盐岩是由单一渗漏通道形成的,生长顺序有待进一步定年证实.烟囱外部6 mm受海水影响较大.研究结果对于恢复南海北部陆坡甲烷渗漏系统、建立冷泉和热泉之间的联系具有一定意义.%Some cold-seep carbonate chimneys collected from the north continental slope of the South China Sea were studied for petrology, mineralogy and geochemistry. The cross cut samples clearly showed distinctive layers. In the inner layer, the carbonate content was higher, and contained higher MgCO3 and bigger minerals, and the minerals crystallized better. While the outer layer contained higher detritus such as Al and Si. The isotopes compositions of δ13C and δ18O were different between the inner and outer layers. For the two studied samples, in the inner layer, δ13C is —40. 8×10-3 and —31. 27×10-3 ,and δ18O is 2. 67×10-3and 2. 00× 10-3, inrn3dividually, and in the outer layer, δ13C is -31. 26×10-3 and -30. 99×10-3, and δ18O is 0. 48×10-3 and 1. 85×10-3, individually. Integrating the former researches near this area, the pattern that δ13C was increasing and δ18O was decreasing from the inner layer to the outer layer. The δ13C and δ18O curves using lmm interval samples range from —27. 50 ×10-3 to — 32. 05 × 10-3 and 0. 78 × 10-3 to 2. 34 × 10-3 from the outer layer to the inner layer individually, which showed the layer structure well. In the outer 6 mm, the δ13C and δ18O had a strongly negative correlation, which correlation coefficient r2 was 0. 996 7. According to the above results, the characteristic of CH4 fluid, its diffusing model and its influences on the sedimentary environment were discussed. The carbonate chimneys studied in this paper were supposed to form in a single channel, but which layer was older needs more dating researches. The outer 6 mm was influenced strongly by the seawater. The results can be used to recover the methane seep system of the northern continental slope of the South China Sea and can also give some relationships between cold seeps and hydrothermal seeps.