ABSTRACT

Whole-rock chemical compositions are significant for igneous petrology, especially volcanic rock studies. ICP-MS is widely used as a conventional method to determine the trace-element compositions using solutions prepared by digestion of rocks. In the past decades, LA-ICP-MS technique has been well developed to measure whole-rock compositions by using pressed powdered pellet or fused glass of rock samples. In this study, trace-element concentrations (Li, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, U and REEs) of basaltic and andesitic rocks were determined by using flux-free fused glasses and LA-ICP-MS. The fused glass was prepared by the direct fusion method where the powdered rock sample is melted with an iridium-strip heater. The heating temperature was manually controlled by monitoring the current and the heating lasted less than 30 sec to achieve melting. We evaluated our routine analytical work performed by measurements of 3 random target positions in each fused glass. The data accuracy was evaluated by measurement of 11 fused glasses created from USGS and GSJ international reference materials (BCR-2, BHVO-2, BIR-1a, JB-2, JA-1 and JGb-1). The homogeneity of our fused glasses is represented by less than 5% relative standard deviation (1σ) in most elements. The mean values of most elements are in agreement with the reference values within 10% deviation. However, Pb concentration cannot be reproduced because of strong volatilization during the melting, and Lu, Hf and Ta are occasionally enriched by the elemental contamination/interference from the iridium-strip heater. 9 fused glasses created from selected petrological samples, such as the Ogasawara volcanic rocks and Oman diopsidite, were also measured to examine our routine measurement work and glass preparation method for a wider range of the trace-element abundance, especially the Cr abundance. The analytical results of 20 fused glasses demonstrate that our method is capable of determining the whole-rock trace-element compositions for petrological study.