Crustal Evolution : Evidence from Ce and Nd Isotope Ratios and REE Abundances

为了弄清陆壳的化学演化，给出了前寒武纪陆相岩石、现代海相沉积物和新生代火山岩的Ce、Nd同位素比值和稀土元素(REE)丰度。得到的结果如下：1.研究结果。西格陵兰太古宙片麻岩和铁质建造的地球化学研究表明，太古宙岩石的来源主要是球粒陨石或似地幔的轻稀土亏损性质，而不是陆相的轻稀土富集性质。这些结果表明，太古宙早期尚未发育轻稀土富集型陆壳。此外，太古宙成铁源区不存在长期的负Ce异常。加拿大地盾晚太古代和早元古代花岗岩的Ce、Nd同位素数据表明，即使在同一优势省，也存在球粒陨石源区和轻稀土富集区。东亚前寒武纪片麻岩的年代学研究表明，本区前寒武纪大陆是由中国东北向外生长的，外加外围省。揭示了中国大陆下地幔在相当长的一段时间内化学成分的不均一性。硅质岩等海洋沉积物中的Ce和Nd同位素比值被证明是其沉积环境的良好指示器。大西洋铁锰结核的Ce和Nd同位素数据显示了轻稀土元素的陆相富集性。另一方面，来自太平洋的铁锰结核具有反映轻稀土贫化的地幔性质的Ce同位素比率，而反映轻稀土富集性的Nd同位素比率则相反。两个大洋样品中Ce同位素组成的差异反映了两个大洋样品中Ce来源的差异。

In order to make clear chemical evolution of continental crust, Ce and Nd isotopic ratios and rare-earth element (REE) abundances were presented for the Precambrian continental rocks as well as recent marine sediments and Cenozoic volcanic rocks. The obtained results are as follows.1. Geochemical studies on the Archean gneisses and iron-formations from West Greenland show that the sources of these Archean rocks had a mainly chondritic or mantle-like light-REE-depleted nature rather than a continental-like light-REE-enriched nature. These results suggest that continental crust with light-REE-enriched nature had not been developed in the early stage of the Archean. Moreover, a long-term negative Ce-anomaly is not implied for the sources of Archean iron-formation.2. Ce and Nd isotopic data on late Archean and early Proterozoic granites from the Canadian Shield suggest that both chondritic source and light-REE-enriched source had existed even in the same Superior Province.3. Geochronological studies on East Asian Precambrian gneisses show that Precambrian continent in this area grew outward from northeastern China by addition of peripheral provinces.4. It is disclosed that upper mantle beneath the Chinese continent had been chemically heterogeneous over a substantial period of time.5. Ce and Nd isotopic ratios in marine sediments such as chert are proved to be a good indicator of their depositional environment.6. Ce and Nd isotopic data of ferromanganese nodules from the Atlantic Ocean show light-REE-enriched continental nature. On the other hand, ferromanganese nodules from the Pacific Ocean have Ce isotopic ratios reflecting light-REE-depleted mantle-like nature in contrast to Nd isotopic ratios with light-REE-enriched nature. The difference of Ce isotopic ratios between samples from two Oceans indicates Ce source difference between them.