Metallogenic environment evolution of the polymetallic crusts and nodules from the South China Sea: Insights from in-situ Pb isotopes and elemental geochemistry

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.