Marine hydrogenetic Fe–Mn crusts on seamounts are known as potential mineral resources of rare earth elements plus yttrium (REY). In recent years, increasing numbers of deposits of Fe–Mn crusts and nodules were discovered in the South China Sea (SCS), yet the enrichment mechanism of REY is yet to be sufficiently addressed. In this study, hydrogenetic Fe–Mn crusts from the South China Sea (SCS) and the Western Pacific Ocean (WPO) were comparatively studied with mineralogy and geochemistry. In addition, we used an in situ REY distribution mapping method, implementing laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and a sequential leaching procedure to investigate the partitioning behavior of REY in the Fe–Mn crusts. The typical Fe–Mn crusts from SCS were mainly composed of quartz, calcite, vernadite (δ-MnO₂), and amorphous Fe oxides/hydroxides (FeOOH). The Fe–Mn crusts from the Central SCS Basin and the WPO contained quartz, δ-MnO₂, FeOOH, todorokite, and phillipsite. Furthermore, geochemical analysis indicated that the typical SCS crusts had a higher growth rate and lower REY concentrations. The LA-ICP-MS mapping results showed that the δ-MnO₂ and FeOOH dominated the occurrence phases of REY in the SCS crusts. Four mineral phases (i.e., easily exchangeable and carbonate, Mn-oxide, amorphous FeOOH, and residual aluminosilicates) in these Fe–Mn crusts were separated by a sequential leaching procedure. In the SCS and WPO crusts, the majority of total REY (ΣREY) was distributed in the Mn-oxide and amorphous FeOOH phases. The post-Archean Australian shale-normalized REY patterns showed that light REY (LREY) and heavy REY (HREY) were preferentially adsorbed onto δ-MnO₂ and FeOOH, respectively. It is noteworthy that ~27% of ΣREY was associated with the residual aluminosilicates phase of the WPO crusts. The La/Al ratios in the aluminosilicates phase of the typical SCS crusts were the values of the upper crust. We conclude that large amounts of terrigenous materials dilute the abundance of REY in the SCS crusts. In addition, the growth rates of Fe–Mn crusts have a negative correlation with the FeOOH-bound and aluminosilicate-bound REY. As a result of the fast growth rates, the SCS crusts contain relatively low concentrations of REY.

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 ³He concentrations and R/R_A values of the SCS crusts are generally higher than those of the SCS nodules, while ⁴He and ⁴⁰Ar 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 ³He concentrations and ³He/⁴He ratios (R/R_A, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the ⁴⁰Ar/³⁶Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low ³He/⁴He ratios and high ⁴⁰Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic ⁴He and ⁴⁰Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic ³He, ⁴He and ⁴⁰Ar 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 ³He 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.

Marine ferromanganese polymetallic crusts and nodules are an important mineral resource widely distributed on the seafloor. They are regarded as strategic reserve resources and have attracted much attention. During the mineralization process, the iron-manganese oxide/hydroxide colloids generated by the oxidation of Fe2+ and Mn2+ in the ambient seawater adsorb metal ions and oxidize the metal ions to a high valence state or form metal complexes to be enriched in iron-manganese minerals. They grow slowly at a rate of several mm/Myr, recording and preserving important information on paleo-ocean and paleoclimate changes. They are ideal objects and response media for studying global ocean evolution and environmental changes and are also helpful in exploring the source-sink process of marine substances. Therefore, based on the precise chronology of polymetallic crusts and nodules from the South China Sea (SCS), this study used in-situ Pb isotope analysis technology to analyze the variation characteristics of Pb isotope composition of polymetallic crusts and nodules, to reveal the growth and mineralization history and environmental evolution of crusts and nodules in the study area. The results show that: (1) The growth ages of SCS polymetallic crusts and nodules are about 1.16-3.46 Ma and an average growth rate of 3.19-6.07 mm/Myr using the 10Be/9Be isotope method. (2) The Pb isotope characteristics of SCS polymetallic crusts and nodules are related to their growth area: the Pb source of crusts and nodules growing in the northern SCS is mainly affected by the input of terrigenous materials; the crusts and nodules growing in the central SCS are less affected by terrigenous materials and are gradually affected by the weathering/alteration of seamount substrates and the input of volcanic activities. (3) Through the coupling of the mineralization chronology framework of SCS polymetallic crusts and nodules with the Pb isotope compositions of their profiles, it is found that the rapid formation and large-scale expansion of the northern hemisphere ice sheet around 3.5 Ma caused the deep water of the SCS to become extremely oxidized, thus promoting the mineralization of SCS crusts and nodules. The tectonic activities in the SCS since 2.8 Ma have had a greater impact on the SCS polymetallic crusts and nodules. Events such as the closure of the Lehe Waterway and the successive closure of the Taitung Waterway have led to the strengthening of the closure of the SCS, the lack of ventilation, and the reduction of oxidation of the seawater, which has slowed the growth rate of polymetallic crusts and nodules. In addition, the uplift of Taiwan and the subsidence of the northern SCS have led to an increase in the input of terrigenous materials, which has led to a decrease in the content of metal elements in the mineralization of polymetallic crusts and nodules. Therefore, the mineralization of SCS polymetallic crusts and nodules is subject to joint control of the tectonic evolution of the SCS and changes in the marine environment such as global climate change.