| 其他摘要 | Diffusion of elements in minerals is one of essential geochemical behaviors, which could affect the information preservation of trace elements and their isotope systematics in minerals, so diffusion of trace elements is widely applied in solving geochemical kinetic problems. Rare earth elements (REEs), as tracers containing important geochemical information (e.g. 147Sm, 143Nd), were used to explore the genesis of ore deposits and rocks. Therefore, it is important to investigate diffusion behaviors of REEs in minerals.
The previous researches on elements (e.g. O, Ar) diffusion in different minerals, mostly adopted the model of porosity, presumed that mineral structure as the only factor influencing the relative mobility of ion in lattice. However, the REEs with small differences in ionic radii could lead to pronounced difference of diffusion behavior, which could not be explained by the porosity model.
In order to find an appropriate method for investigating diffusivities of REEs in minerals, the previous experimental data have been summarized and analyzed, and the effects of crystal-chemical factors on the mobility of ions in minerals are also considered in this doctoral dissertation. On these bases, a multivariate model is constructed. It is revealed that, diffusion parameters of REEs in minerals calculated by multivariate model, are in a good agreement with experimental results.
Sm-Nd dating method on scheelite has great potential in determining the exact time of hydrothermal mineralization, in order to assess the reliability of this approach, the following methods are adopted in this doctoral dissertation.
By employing theoretical study, the closure of Sm-Nd isotope systematics has been investigated in scheelite. The diffusion parameters and Arrhenius relations of Sm and Nd in scheelite under the condition of dry and 1 atm are obtained:
Nd：D＝4.00exp（－438kJmol-1/RT）cm2/s
Sm：D＝1.85exp（－427kJmol-1/RT）cm2/s
Based on above diffusion data, the closure temperatures and the preserved conditions of Sm-Nd isotope systematics in sceelite are discussed in this dissertation.
In addition, the Woxi Au-Sb-W deposit is studied to test the reliablitity of scheelite Sm-Nd dating. The results of Sm-Nd dating of scheelite from different orebodies coincide well with Ar-Ar ages, tectonic evolution and geochronological data of some Au (Sb-W) deposits in western Hunan.
Some important conclusions could be drawn in this dissertation as follows:
1. For the first time, the multivariate model for REEs diffusion in different minerals is obtained, which provides a suitable approach for studying diffusion of REEs.
2. Diffusion of REEs in scheelite is revealed for the first time, diffusion parameters and Arrhenius relations of REEs in scheelite are obtained.
3. The diffusion rates of both Sm and Nd in scheelite is very slow, thus the closure temperatures of Sm-Nd in this mineral are relatively high, and original isotope information in scheelite should be preserved under all geological conditions except the most extreme thermal condition.
4. The successful dating of scheelite Sm-Nd isochron for the Woxi Au-Sb-W deposit, further demonstrates that Sm-Nd isotope dating of scheelite is reliable. |
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