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

Abstract

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 3 He concentrations and R/R A values of the SCS crusts are generally higher than those of the SCS nodules, while 4 He and 40 Ar 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 3 He concentrations and 3 He/ 4 He ratios (R/R A , 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the 40 Ar/ 36 Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low 3 He/ 4 He ratios and high 40 Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic 4 He and 40 Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic 3 He, 4 He and 40 Ar 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 3 He 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.isotopes provide important geological information for the research of modern ocean circulation, paleoceanography, seafloor hydrothermal and cold brine system and ocean/atmosphere gas exchange.Early studies of marine noble gases are mainly concentrated in solving geochemical problems, such as tracing the excess He in the seawater, which is likely produced by U-Th series radioactive decay in the marine sediments [1,2]. After the discovery of mantle-originated helium [3] and 3 He originated near the sediment surface (via 3 H decay in the sea) [4], people began to use noble gases to trace the source of submarine hydrothermal fluids, crust/mantle material cycle, constrain the formation process and thermodynamic mechanism of seamount systems [5][6][7][8][9][10][11] and to explore the formation and cycle of the ocean currents (e.g., [12][13][14]).Some studies have explored potential application of He isotopes...

He-Ar Isotopes and Trace Gas Compositions of Fluid Inclusions in Massive Sulphides from the Yushui Copper-Polymetallic Deposit, South China: Metallogenic Implications

“…To address the above-mentioned questions, the application of noble gas (especially helium and argon) isotope geochemistry is key to defining the origin of ore-forming fluids. In particular, as a monatomic chemically inert gas ( 3 He behaves extremely conservatively during mixing of mantle-/crustal-derived fluids with meteoric water), helium is considered to be one of the most sensitive geochemical tracers for such studies and has distinct isotopic signatures in different types of geological reservoirs [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. For ancient polymetallic ore deposits, the He-Ar isotopic compositions of hydrothermal fluids are faithfully preserved when trapped as fluid inclusions by mineral phases such as pyrite, chalcopyrite, sphalerite, or galena, and thus can be widely used to reveal the degree of fluid-rock interaction, mantle degassing or magmatic activity [35][36][37][38][39][40][41][42][43][44][45][46][47].…”

Distribution of Rare Earth Elements plus Yttrium among Major Mineral Phases of Marine Fe–Mn Crusts from the South China Sea and Western Pacific Ocean: A Comparative Study

“…In addition, a number of mining areas with the potential for exploitation were also found in the Atlantic Ocean [8,11], Indian Ocean [12,13], and Arctic Ocean [14]. In addition to the open oceans, some Fe-Mn crust deposits were also discovered near the continental margin, such as the Baltic Sea [15], the California continental margin [2,16,17], and the South China Sea (SCS) [18][19][20][21].…”