ELECTRON-MICROPROBE ANALYSIS OF CA IN OLIVINE CLOSE TO GRAIN-BOUNDARIES - THE PROBLEM OF SECONDARY X-RAY-FLUORESCENCE

Analysis of Ca in olivine grains close to boundaries with Ca-rich phas es is inherently problematic because of the effects of secondary X-ray fluorescence of Ca from the adjoining Ca-rich phase. Commercial fluor escence-correction methods assume chemical homogeneity throughout the entire interaction volume and yield enhanced Ca concentrations for the olivine in such cases. We have determined the effects of Fe content o f the olivine and Ca content of the adjoining phase on the magnitude o f secondary fluorescence effects during electron microprobe analysis o f Ca in olivine. Significant errors in Ca determination were observabl e at horizontal beam-interface distances ranging from 30 mu m for an F e-free olivine-diopside couple to 70 mu m for an olivine (Fo(90.7))-ca lcite couple. In general, the beam-interface distance required to supp ress secondary fluorescence of Ca X-rays from the adjoining phase incr eases with increasing Fe content of the olivine and Ca content of the neighboring phase. Analysis of an olivine (Fo(90.7)) wedge mounted on a standard thin-section glass slide (5.06 wt% Ca) showed significant s econdary fluorescence of Ca to a wedge depth approaching 20 mu m. It i s recommended that olivine grains be separated and remounted in a Ca-f ree matrix for the determination of Ca concentrations in olivine by el ectron microprobe analysis.