超基性岩化学前处理方法探索及超微量亲石/亲硫元素含量同位素分析和在西藏班公湖-怒江蛇绿岩带地幔橄榄岩中的应用研究

超基性岩，镁含量高，样品酸溶后的蒸发过程中，会形成相当量的氟化镁难溶物，严重影响了后化学处理；此外，这类样品富含铬铁矿、尖晶石等难熔矿物，影响样品的分解。难溶氟化物和难熔矿物的分解相关样品化学前处理，均影响超微量元素分析数据的质量。作为获取准确分析数据的第一步，合理正确的样品分解方法非常重要，所以本项目拟针对超基性岩样品的分解进行实验，探索样品完全分解的条件参数。基于全溶样品，进一步实验ppb量级亲石和亲硫元素的化学分离富集，建立准确的超微量元素(如稀土REE、铂族PGE)含量和同位素(Sr-Nd-Hf-Os)分析方法。新建方法应用于西藏班公湖-怒江缝合带地幔橄榄岩样品，测定REE、Re、PGE和大离子高场强元素Nb、Ta、Zr、Hf等元素含量和Sr-Nd-Hf-Os同位素比值，揭示相关地幔岩石地球化学特征，探讨青藏高原特提斯洋地幔演化过程。

Ultra-mafic rocks have considerable Mg, which can change to MgF2 during chemical preparation after their acid dissolution. MgF2 is a slightly soluble compound and difficult to be decomposed. As a result, it has a strong effect on the subsequent chemical treatment. In addition, these ultra-mafic rocks generally consist of refractory minerals such as chromite and spinel, which are difficult to be dissolved and have effects on the decomposition of samples. Certainly, the analytical data found for ultra-trace elements such as rare earth elements in the rocks are affected by both insoluble MgF2 and refractory minerals. As the first step, the decomposition of samples is very important to obtain high quality data. Therefore one of our aims is firstly to investigate and find out parameters for total decomposition of ultra-mafic samples. After samples are decomposed based on the parameters, the contents of ultra-trace elements such as lithophile REE and chalcophile PGE will be accurately determined with combination of chemical separation and preconcentration. Via the application of this method to the ultra-mafic samples, Bangong-Nujiang suture zone, Tibet, both elemental contents (e.g. REE, Re, PGE, Nb, Ta, Zr, Hf) and isotope compositions (Sr-Nd-Hf-Os) will be analyzed. The analytical data will reveal the geochemical characteristics for further exploring the evolution of Tibet tethyan oceanic mantle.