Polymetallic nodules and crusts are two of the most important mineral deposits in the ocean.They are rich in rare earth elements (REEs),iron,manganese,copper,cobalt,nickel,and other useful metals.This paper discussed the analysis of 25 nodule and crust samples collected from the South China Sea,the Pacific,Atlantic,and Indian Oceans.The samples were analyzed for REE content by ICP-MS/AES. The average REE concentration was found to be 1096.96×10-6 in the nodules and 1623.88×10-6 in the crusts.Both of these values are much higher than those recorded in Earth's dry-land crust and sedimentary rocks.This REE enrichment is mainly controlled by the absorption of ferromanganese oxides and clay minerals in the nodules and crusts and the high levels of REEs in seawater and sediments.High cerium enrichment in the nodules and crusts may lead to more effective exploitation of REEs in the future.
Two kinds of basalts with different chemical compositions were obtained in the Sanshui Basin,they share the same eruption period. Although they have significant differences in major and trace element concentrations,their isotopic ratios suggest a similar magma source. Based on mineral geochemical analysis,orthopyroxene with reaction rim and zoned clinopyroxene are observed in Zidong(ZD)basalts,but not found in Wangjiegang(WJG) basalts. Linear scanning of these minerals suggests compositional variation between within-plate tholeiite and within-plate alkali-basalt. Presence of a doublelayer magma chamber under the Sanshui Basin,and occurrence of magma mixing between upper and lower chambers is proposed. This magma mixing leads to different chemical composition and mineral constitution of two kinds of basalts,as well as reaction rim and compositional variation of minerals in ZD basalts.
In the northern South China Sea,the accumulation of enormous quantities of terrigenous sediment during Cenozoic rendered well-developed polymetallic nodules very rare.In this study,we analyzed a polymetallic nodule from the northwestern continental margin of the South China Sea using microscopic mineralogical observation,electron probes,X-ray diffraction(XRD),ICP-MS,and Be isotope dating.We found the nodule's shell layers rich in different types of microstructures,including columnar,laminar,stack-like,petal-like,and porphyritic structures.The major mineral components of the nodule are MnO2.Unlike nodules from the eastern Pacific basin,this nodule has high contents in Fe,Si,Al,and REEs but low contents in Mn,Cu,Co,and Ni.The Mn/Fe ratio is also low and the average REEs content is 1370.4 ppm.There is a strong positive anomaly of Ce;and the Be(beryllium) isotope dating shows the initial time of growth of the nodule to be about 3.29 Ma.The inner compact layer formed from 3.29 Ma to about 1.83 Ma.The laminar and stack-like structures and the low contents of the terrigenous elements such as Fe,Si,REE,and Al indicate the paleoceanographical environment with weak undersea currents and favorable oxidizing conditions.From 1.83 Ma to 0.73 Ma,the growth rate of the nodule increased by about 3%;the microstructures formed during this period are stack-like and columnar.The contents of Si and Al are increased by nearly 10%,indicating an increase of terrigenous sediment input in the northern South China Sea.The content of Ce is decreased by about 16%,indicating a significant weakening of the oxidizing conditions at the seabed.From 0.73 Ma to 0.69 Ma,the growth rate of the nodule rapidly rose up to 8.27 times that of the nodule's average growth rate,and the contents of Fe,Al,and REEs in the layer also increased,forming a loose layer characterized by oolitic,granular,porphyritic,and petal-like structures,indicating the paleoceanographical environment with a high sedimemtation rate and abundant supply of terrigenous sediment in the
