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 phases is inherently problematic because of the effectsof secondary X-ray fluorescenceofCa from the adjoining Ca-rich phase. Commercial fluorescence-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 effectsof Fe content of the olivine and Ca content of the adjoining phase on the magnitude of secondary fluorescence effects during electron microprobe analysis of Ca in olivine. Significant errors in Ca determination were observable at horizontal beam-interface distances ranging from 30 /.Lmfor an Fe-free olivine-diopside couple to 70 /.Lmfor an olivine (Fogo.7)-calcite couple. In general, the beam-interface distance required to suppress secondary fluorescence of Ca X-rays from the adjoining phase increases with increasing Fe content of the olivine and Ca content of the neighboring phase. Analysis of an olivine (Fogo.7) wedge mounted on a standard thin-section glass slide (5.06 wt% Ca) showed significant secondary fluorescence of Ca to a wedge depth approaching 20 /.Lm.It is recommended that olivine grains be separated and remounted in a Ca-free matrix for the determination of Ca concentrations in olivine by electron microprobe analysis.