铌与钽的某些地球化学问题

Some aspects on geochemistry of Nb and Ta

针对地球圈层之间Nb、Ta质量不平衡的难题,较系统收集分析了我国基性岩墙(脉)群、太古宙基性火山岩、碱性岩、大火成岩省火山岩,俯冲带中富Nb玄武岩、高压变质岩、花岗岩类的Nb、Ta含量、比值、相关同位素组成以及Nb、Ta实验地球化学资料。这些资料表明,Nb、Ta在这些岩类中的分布呈现非常不均匀变化,除花岗岩外,上述岩石的Nb/Ta比值均高于或近于球粒陨石值17.5。花岗岩中的幔源斜长花岗岩及与裂谷、热点环境有关的碱性花岗岩Nb/Ta比值近于地球平均值,而普通S型及I型花岗岩,特别是高演化花岗岩,Nb/Ta比值均明显低于球粒陨石(<10),甚至呈现Nb/Ta≤1。这些资料表明,在不均匀的地幔中可能存在呈布丁状分布的Nb/Ta比值高于球粒陨石的储源。金红石的稳定性及Nb、Ta分配系数实验资料不支持地球深部存在高Nb/Ta比值的含金红石榴辉岩。

Considering a mass imbalance exists in Earth for Nb and Ta, the contents of Nb and Ta and the Nb/Ta rations in mafic dike swarms, Archean basic volcanic rocks, alkaline rocks, volcanic rocks of large igneous province, Nb-enriched basahs, ultrahigh-pressure metamorphic rocks and different types of granites in China are systematically investigated. These data have shown obvious fractionation of Nb and Ta. These rocks, except for granites, have superchondritic or chondritic Nb/Ta ratios （17.5）. Subchondritic or approximate chondritic Nb/Ta rations are observed in mantle-derived plagioclase granites and alkaline granites related to the tectonic settings of rift or hot sports. General granites possess very low Nb/Ta （〈 10）, even Nb/Ta≤ 1 for highly evolved granites. These data have revealed that some reservoirs with superchondritic Nb/Ta ratios may reside as budding-form in the mantle. The experimental data on the stability of ruffle and ruffle/melt Nb and Ta partitioning do not support the view that subducted ruffle-bearing eclogite is a superchondritic Nb/Ta reservoir in the deep mantle. K