Approach To Geochemical Characteristics And Minerogenetic Environment Of Polymetallic Nodules From The Northern Continental Margin Of The South China Sea

The polymetallic nodules collected from the northern continental margin of the South China Sea are used in this study. Samples of diferent nodules which are from the Pacific and the Indian Ocean are discussed in details with mineralogical and element geochemical method. This study showed great value on the theoretics and the appliance.The study used various observations and analytical techniques such as electron microprobe analysis, X- ray difraction ( XRD). Elemental content data of polymetallic nodules are obtained by ICP- AES and ICP-MS techniques. The chronology is mostly based on the calcareous nannofossils from the sample. The studies are including of elemental composition, content and distribution, the ratio among different elements, elemental assemblage, triangular diagram analysis and REE distribution pattern. The minerogenetic environment of the polymetallic nodules are discussed in this article. The following results have already been achieved:1. According to internal structure features, the microstructure can be classified into 12 different types in the polymetallic nodules. The XRD, ICP-AES and ICP-MS data show different characteristics from different areas. Sample of the South China Sea is rich in Fe, Si, and∑REE, and is poor in Mn, Cu, Co, Ni and Mn/Fe. Elements correlation of w(Mn)-w(Fe)-the w(Cu+Ni)×10 suggest that the origin of the sample may be of hydrogenic.2. The chemical compositions of polymetallic nodules which are collected from the South China Sea, northeastern Pacific Basin, northwestern Pacific Seamount and central Indian Ocean Basin are studied in this article. The sample of the South China Sea has many particular characteristics on main elements, trace elements and rareearth elements (REE), and is easily different from the oceanic nodules and the nodules which are obtained from the Baltic and the Okinawa Trough. The sample has lower rates on Cu, Co, Ni, and Mn/Fe (0.73). The REE expresses very special characteristics of the sample. It has higher content than oceanic nodules (1.8). The heavy REE (HREE) depleted relative to the light REE (LREE) very obviously (19.54), because the HREE have formed more stable complexes in seawater than LREE. The distinct enrichment ofδCe and unusual negative ofδEu indicates strong oxidize oceanic environment of the study areas.3. According to the correlated coefficient analysis, there are positive relations among Mn, Mg, Cu, Ni, Al and Ca, also Fe, Co, Zn, Ti, Pb, Sr and∑REE. There are negative relations between Mn+Fe and Mg+Al+Si contents, Cu+Ni+Co and Fe+Mn contents. The content ration of Fe increases from the inner crust to the outer one, with that of Cu and Ni decreasing in different ways. It shows that the growth of the nodules was affected by the environmental fluctuations and changing supply of the terrigenous sediments and Mn, Cu, Ni, and Ca. The nodules are typically hydrogenetic in marginal sea environment.4. According to the chronology correlation of calcareous nannofossils, the growing age of the polymetaic nodule has been identified. The geological age is limited to 1.7Ma, and the growth rate is about 15.38 mm/Ma. The nodule may represents a new type which grows fast in high deposition velocity environment.5. Based on the detailed study of the geochenistry and growth rate, the minerogenetic environment is discussed in this paper. The nodule can grow fast because the north of the South China Sea has better conditions on forming mechanism and environments, such as special geography position, ample mineral supply and good paleoceanic conditions. In addition, its growth process can record a lot of palaeoceangraphic, palaeoclimatological and palaeoenvironmental information. This study will achieve in explaining regional environmental changes even global environmental evolution.