RAPID QUANTITATIVE ANALYSES OF REE AND TRACE ELEMENTS IN 1:10 XRF GLASSBEAD USING LA-ICPMS

Analysis of trace elements in whole rock samples is indispensable to evaluate the metal potential of rare-metal mineral deposits. Recent progress in the analytical tools enabled us to determine the trace-element abundances from geochemical rock samples. Commonly, compressed powder-pellet analyses using XRF and/or acid decomposed sample analyses using ICP-MS have been widely employed because their reliability of the data as well as user friendly operations. However, with the XRF method, analytical sensitivity is sometime not high enough to determine trace elements, such as rare earth elements (REE). Acid digestion technique, takes time and needs great care for the complete digestion or dissolution of some heavy-minerals and accessory minerals, such like zircon, apatite or monazite which are sometimes main REE-containers in the rocks.

We applied glassbead-ablation method was for analyses of whole rock composition using LA-ICPMS. In this study, the glassbeads were made of 0.5g of sample and 5.0g of lithium-tetraborate for accurate XRF analysis. The present method has several advantages: 1) higher sensitivity relative to XRF method, 2) incomplete dissolution of heavy minerals can be avoided by alkali-fusion decomposition, 3) easier sample preparation results in large number of the analysis in a shorter time. We developed this methods by following approaches: 1) femtosecond laser-ablation was applied for efficient sample introduction, 2) overall controlling software was designed for establish a cooperative system of the laser, sample stage and ICP-MS, 3) sample cell and holder were also designed for enhance the efficiency. A calibration curve was made by using geochemical reference samples of Geological Survey of Japan.

These improvements enabled us to analyze glassbeads within 3 min/sample. 15 samples were placed in the sample cell at a setting, and analyzed point were efficiently fed by using preset feature within 10 sec/point. In this method, uncertainty of the results were given within 10% for Ca, Ti, Mn, Fe, Zn, Rb, Sr, Y, Zr, Ba, REEs, Hf, Th, U and 20% for Mg, P, Sc, Cr, and Ge.