Geochemical characteristics and geological significance of cold seep carbonates from the Haima cold seeps,northern South China Sea
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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