南海北部海马冷泉碳酸盐岩地球化学组成特征及地质意义

稀土元素和Nd同位素组成是示踪冷泉碳酸盐岩形成环境以及流体来源的重要工具。本研究对采集于南海北部琼东南盆地海马冷泉区两个站点的筒状和含贝壳冷泉碳酸盐岩展开一系列元素和Nd同位素组成研究。筒状冷泉碳酸盐岩和含贝壳冷泉碳酸盐岩的Sr/Ca比值分别为0. 01~0. 02和0. 01~0. 03，而Mg/Ca比值分别为0. 03~0. 09和0. 01~0. 10，指示两种类型的冷泉碳酸盐岩中碳酸盐矿物分别以文石和高镁方解石为主，其中前者指示了较高通量甲烷渗漏和较高沉淀速率的形成环境。两种类型冷泉碳酸盐岩具有相似的稀土元素总量（ΣREE），分别为36. 9×10-6~41. 8×10-6和36. 0×10-6~53. 5×10-6，且均以富集轻稀土元素（LREE）和亏损重稀土元素（HREE）（LREE/HREE分别为11. 2~11. 6和10. 0~11. 4）为特征，二者均无明显的铕异常（δEu分别为1. 03~1. 05和1. 00~1. 03）和铈异常（δCe分别为0. 97~0. 98和0. 92~1. 01），指示其应形成于缺氧环境。筒状冷泉碳酸盐岩和含贝壳冷泉碳酸盐岩均具有相对较低的143Nd/144Nd同位素比值（分别为0. 512066~0. 512096和0. 512059~0. 512087），计算得到εNd值分别为10. 8~10. 6和11. 3~10. 7，且εNd和1/Nd具有明显的负相关性，指示形成两种类型冷泉碳酸盐岩的流体中Nd可能是混合来源，其中绝大部分Nd来源于海水和沉积物的水岩反应释放出的富集143 Nd流体。 Rare earth element (REE) compositions and Nd isotopic signatures are important tools for tracing the formation environments and fluid sources of cold-seep carbonates. This study investigates the elemental and Nd isotopic compositions of tubular and shell-bearing cold-seep carbonates collected from two sites within the Haima cold seep in the Qiongdongnan Basin, northern South China Sea. The Sr/Ca ratios of tubular and shell-bearing carbonates range from 0. 01 to 0. 02 and 0. 01 to 0. 03, respectively, while their Mg/Ca ratios arc 0. 03 to 0. 09 and 0. 01 to 0. 10, respectively. These ratios suggest that the carbonate minerals in shell-bearing carbonates arc primarily aragonite, while those in tubular carbonates are predominantly high-Mg calcite. This difference indicates that shell-bearing carbonates formed under conditions of relatively high methane leakage flux and precipitation rates. Both types of cold-seep carbonates have similar total REE (ΣREE) concentrations, ranging from 36. 9 X 10-6to 41. 8 X 10-6for tubular carbonates and 36.0X10-6to 53. 5X10-6for shell-bearing carbonates. They are characterized by enrichment in light REEs (LREEs) and depletion in heavy REEs (HREEs), with LREE/HREE ratios ranging from 11. 2 to 11. 6 and 10. 0 to 11. 4, respectively. Additionally, both types lack significant Eu anomalies (δEu = 1. 03 ~ 1. 05 and 1. 00 ~ 1. 03, respectively) and Cc anomalies (δCc = 0. 97 ~ 0. 98 and 0. 92~1. 01, respectively), suggesting formation in anoxic environments. Both tubular and shell-bearing cold-seep carbonates exhibit relatively low 143Nd/ 144Nd ratios, ranging from 0.512066 to 0.512096 and 0. 512059 to 0. 512087, respectively. The calculated ϵNdvalues range from - 10. 8 to - 10. 6 for tubular carbonates and - 11. 3 to - 10. 7 for shell-bearing carbonates. A notable negative correlation between ϵNdand 1/Nd suggests that the Nd in the fluids responsible for forming these carbonates is likely derived from mixed sources, with the majority of Nd predominantly originating from 143Nd-enriched water, which was released from the seawater-sediment interaction. © 2024 Geological Society of China. All rights reserved