Scanning Electron Microscopy


Considerably improved suppression of molecular ions in secondary ion mass spectrometry (SIMS) spectra of nonconductor minerals has been obtained using a CAMECA IMS-3f ion microscope with unconventional operating conditions [so-called specimen isolated (SI) conditions]. In a zircon spectrum close to forty elements are positively identified and molecular ions such as oxides and hydrides have very low intensities. Thus, with a 28Si+ intensity of 106 cps, the Si+/Si0+ ratio is 105, and the 30SiH+ intensity is low enough to enable quantitative analysis using 31P+ down to 0.01 wt % P2O5 in a silicate glass matrix. The SI conditions enable us to follow major, minor and trace element concentrations across a complex alteration zone such as a sphene/hornblende contact. Isotope ratios show reasonable agreement with natural isotopic abundances, but relatively large "kinetic energy" induced isotopic fractionation is observed due to our analysis of high kinetic energy secondary ions. For zircon, and sphene samples, the isotope fractionation plotted against the mass ratios of the isotopes shows a linear dependence.

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