MICROPROBE ANALYSES OF REE AND HALOGENS IN ACCESSORY MINERALS,ESPECIALLY FOR APATITE

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The halogene and rare-earth elements in accessory apatite,allanite and zircon from some dome dacites and granite have been investigated using the electron microprobe. For these purpose,the system and the method by use of a JXA-50A microanalyzer were improved in performing microprobe analysis at low concentrations of minor element,such as,fluorine,chlorine,strontium,yttrium, REE,zirconium,hafnium and thorium. Methods of spot analysis were applied for analysis of very fine apatite microlite in groundmass of dacitic volcanic rocks and for that of zoned allanite and zircon crystal in some granitic rocks.Pulse height analyzer were put to practical use for improvement in peak/backbround intensity ratio. The ZAF correction methods were deveroped for calculation using intensity data of characteristic X-ray at different accelerating voltages. Intensity data of major elements were obtained under a operating condition of 2x10^<-8>A specimen current at 15 kV accelerating voltage. Analyses of fluorine,chlorine and the other minor elements were made under 8x10^<-8>-1.5x10^<-7>A at 20-25 kV accelerating voltages. The detection limits thus achieved were as follows; F 200 ppm,Cl 40 ppm,Y 210 ppm,La 100 ppm,Ce 100 ppm,Nd 230 ppm,Er 120 ppm,Yb 120 ppm,Th 200 ppm.Resultant analyses of apatite microlite in groundmass of the dome dacites indicate that halogene contents of apatite were useful indicator of degassing of magma. In granitic rocks high concentrations of REE in these accessory minerals indicate that they contain the greater proportion of whole-rock REE. Allanite dominantly plays important role in fractionation of light REE in evolution of felsic magma.