SIMS ANALYSES OF RARE-EARTH ELEMENTS IN NATURAL MINERALS AND GLASSES - AN INVESTIGATION OF STRUCTURAL MATRIX EFFECTS ON ION YIELDS

An investigation of the influence of matrix structure upon secondary ion signals for rare earth elements (REEs) in various silicates (amphiboles, clinopyroxenes. garnets, and plagioclases) was carried out by means of conventional energy filtering technique (CEF). Such matrices were analysed both as natural crystals and fused glasses. Of the selected REEs for the different minerals, only La showed an intensity relatively higher than Si in glassy amphiboles and augites. The hypothesis proposed of an ionization behaviour of La that differs with respect to other REEs in silicates has not been confirmed in plagioclases. All other relative REE ion yields from crystals and glasses agreed within the statistics of measurement for all investigated matrices. Moreover, any difference between crystal and glass REE ionization appeared not to be attributable to varying charging conditions during analysis under O- primary beam bombardment. Laboratory contamination during glass preparation and/or the presence of residual light REE-enriched micron-scale accessory phases (e.g., apatite) could be responsible for the higher La/Si signal in augite and amphibole glasses. Results obtained show that the efficiency of REE ionization compared with that of Si, generally is not affected by the sample structure as far as high energy secondary ions are concerned; therefore, glasses obtained by fusing silicate minerals are suitable for analytical work and can be employed to calibrate REE ion signals for quantitative purposes.