南海北部冷泉碳酸盐岩的矿物、岩石及地球化学研究进展

Progresses on petrology, mineralogy and geochemistry of cold seep carbonates in the northern South China Sea

南海北部陆坡海底冷泉碳酸盐岩广泛发育,已从30个站位采集到冷泉碳酸盐岩。南海北部冷泉碳酸盐岩呈结壳状、结核状、烟囱状、角砾状、块状等,主要自生矿物有文石和方解石,部分站位样品有白云石和铁白云石及少量菱铁矿。冷泉碳酸盐岩具有较轻的碳同位素组成特征,西沙海槽海域、神狐海域、东沙西南海域、东沙东北海域和台西南海域碳酸盐岩的δ13CPDB分别为29.6‰—13.3‰(平均21.0‰),40.4‰—38.7‰(平均39.8‰),36.1‰—18.2‰(平均26.8‰),61.4‰—32.8‰(平均49.3‰)和57.6‰—35.7‰(平均48.3‰),显示了较大的变化范围,反映了复杂的碳源。冷泉碳酸盐岩的δ18OPDB为0.4‰—7.5‰,同样显示了较大的变化范围,反映可能与富集18O的地层水和/或天然气水合物分解水有关。神狐海域及东沙东北部九龙甲烷礁海域冷泉碳酸盐岩的稀土元素页岩标准化配分模式既具有Ce正异常,同时也有Ce负异常特征,说明冷泉碳酸盐岩形成过程中可能存在氧化还原条件的变化。南海北部陆坡不同海域,甚至同一站位的冷泉碳酸盐岩均表现出不同的矿物、同位素及元素组成特征,可能反映了冷泉渗漏流体和天然气水合物稳定状态的差异。

Cold seep carbonates are known to be commonly developed in the northern South China Sea. Authigenic carbonate samples were collected from thirty sites in the region. These carbonates consist of concretions, nodules, chimneys, fragments or massive blocks. Mineralogically, the carbonates are dominated by aragonite and Mg calcite. However, a certain amount of dolomites are present in some samples. The carbon isotopic compositions （813CpDB） of the carbonates range from -29.6%0 to -13.3%o in the Xisha samples, from -40.4%o to -38.7%o in the Shenhu samples, from -36.1%o to -18.2%o in the southwestern Dongsha samples, from -61.4%o to -32.8%o in the northeastern Dongsha samples, and from -57.6%o to -35.7%o in the southwestern Taiwan samples. The variable 613C values indicate complex carbon sources that include 13C-depleted biogenic and thermogenic methane. A similarly large variability in 818OpDB values （0.4%~ to 7.5%o） demonstrates 180-enriched fluids possibly associated with dissociation of locally abundant gas hydrate. The shale-normalized REE （rare earth element） patterns of the carbonates from Jiulong methane reef in northeastern Dongsha and from Shenhu show both positive and negative regions and from the regions, even the same Cerium anomalies, suggesting that the redox conditions changed significantly. Samples from different same sites show variations in mineralogical, isotopic and elemental features, indicating that different sites, developed various seep fluids and gas hydrate stability.