Helium and Argon Isotopes in the Fe-Mn Polymetallic Crusts and Nodules from the South China Sea: Constraints on Their Genetic Sources and Origins

In this study, the He and Ar isotope compositions were measured for the Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS), using the high temperature bulk melting method and noble gases isotope mass spectrometry. The He and Ar of the SCS crusts/nodules exist mainly in the Fe-Mn mineral crystal lattice and terrigenous clastic mineral particles. The results show that the He-3 concentrations and R/R A values of the SCS crusts are generally higher than those of the SCS nodules, while He-4 and Ar-40 concentrations of the SCS crusts are lower than those of the SCS nodules. Comparison with the Pacific crusts and nodules, the SCS Fe-Mn crusts/nodules have lower He-3 concentrations and He-3/He-4 ratios (R/R-A, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the Ar-40/Ar-36 ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low He-3/He-4 ratios and high Ar-40 concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic He-4 and Ar-40 contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic He-3,He-4 and Ar-40 produced by decay of U, Th and K have no effect on their isotope compositions. Thus, the SCS crusts/nodules inherited the noble gases characteristics of their sources. Helium and Ar isotope compositions in the SCS Fe-Mn crusts and nodules reflect the product of an equilibrium mixture between air-saturated seawater and radiogenic components during their growth, while the partial He-3 excess in some SCS samples may represent a little mantle-derived origin. The different He and Ar isotope compositions of the Fe-Mn crusts and nodules between the South China Sea and the Pacific Ocean are due to their different sources and genetic processes. The characteristics of He and Ar isotope compositions in the SCS polymetallic crusts and nodules are similar to the properties of hydrogenetic Fe-Mn oxide/hydroxide precipitates, which reflects mainly the product of an equilibrium mixture between air-saturated seawater and radiogenic components.