南海北部陆坡九龙甲烷礁冷泉碳酸盐岩沉积岩石学特征

Petrographic characteristics of authigenic carbonates from Jiulong methane reef of northern South China Sea

本文对南海北部陆坡九龙甲烷礁采集的两个冷泉碳酸盐岩样品TVG-1和TVG-11进行了矿物学、岩石学和碳氧同位素分析研究。X-射线衍射分析(XRD)结果表明,自生矿物主要为文石、白云石和高镁方解石等,碎屑矿物含量较少。TVG-11中存在大量的文石,而TVG-1只保留了已转化为方解石的文石假象,据此可以推断TVG-1的形成要早于TVG-11。扫描电镜(SEM)显微结构特征显示,自生矿物主要是通过细菌的厌氧氧化作用,直接从冷泉中沉淀出来的。碳、氧同位素测试结果显示,碳酸盐岩具有强13C亏损和一定程度的18O富集的特征,TVG-1和TVG-11的δ13C值分别为-46.22‰和-52.88‰,均低于-40‰,表明碳源于微生物作用的甲烷厌氧氧化,是指示该海域存在冷泉的重要证据;TVG-1和TVG-11的δ18O值分别为3.19‰和4.07‰,存在一定程度的18O富集。碳氧同位素和矿物的显微结构特征都说明九龙甲烷礁碳酸盐岩是烃类渗漏微生物作用而形成的自生碳酸盐岩,可能是天然气水合物分解引起的沉积,显示该区存在水合物的可能性很大。

The authors report on the mineralogical features and carbon-oxygen isotope composition of the authigenic carbonates build-up of Samples TVG 1 and TVG-11 from Jiulong methane reef in the South China Sea. The results by XRD confirm that the samples are composed of miscellaneous carbonates, including aragonite, Mg-calcite and dolomite, while detrital minerals are less abundant. In Sample TVG-1, aragonite is replaced by calcite and the pseudomorph of the former is observed, suggesting that this sample was formed earlier than the other because phase transformation of aragonite to calcite has occurred. SEM observation reveals that micro-scale acicular aragonite is usually associated with bio-filaments which may be extra-cellular polymeric substances produced by microorganisms. These observations confirm that carbon- ates precipitation is related to heterotrophic microbial communities fueled by hydrocarbon seepage only having weak activity currently. Stable isotope composition of the carbonates is characterized by strong 13C depletion, with a δ^13C value of --46.22‰ and --52.88‰ for TVG-1 and TVG-11 respectively, indicating that the carbon source is most probably related to anaerobic oxidation of methane. Another distinct feature of the carbonates is its enrichment of δ^18O ＋3.19‰ and ＋4.07‰ for TVG-1 and TVG-11, respectively. 13C depletion and δ^18O enrichment along with the micro fabric textural evidence suggest that the Jiulong carbonates build-up might have been precipitated from the fluids originating from gas-hydrate decomposition, which indicates that this area may have gas hydrate resources.