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 are 0.03 to 0.09 and 0.01 to 0.10,respectively.These ratios suggest that the carbonate minerals in shell-bearing carbonates are 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 36.9×10-6 to 41.8×10-6 for tubular carbonates and 36.0×10-6 to 53.5×10-6 for 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(8Eu=1.03～1.05 and 1.00～1.03,respectively)and Ce anomalies(δCe=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 eNd values 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 eNd and 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.
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