Potentials of Electron Probe Microanalysis in Studying Zircons

Abstract

Zircon is a widespread accessory mineral of magmatic rocks; it attracts interest because of its high stability to chemical and mechanical erosion within the geochemical cycle. The high stability of zircon makes it suitable for interpreting the genesis of magmatic rocks [1, 2] and determining their age by isotope dilution [3], secondary ion mass spectrometry [4], and the Th-U-Pb method [5], which can be used for determining Th, U, and Pb in zircon [6,7]. In these terms, valuable information can be derived from both morphological features and geochemical parameters, primarily from the distribution of the major and impurity elements in the crystal [9].Most of the data on zircon geochemistry are usually obtained by analyzing relatively large samples using the methods of nuclear physics (neutron activation analysis), emission spectroscopy, atomic absorption spectroscopy, inductively coupled plasma emission spectroscopy, and others. All these methods are characterized by good detection limits (10 -3 to 10 -7 %), while determining total impurity elements. Spark mass spectrometry [10] is also characterized by insufficient locality properties (more than 100 µ m spot) as compared to electron probe microanalysis and ion microprobe analysis. The latter method, although having unique analytical capabilities [4], is poorly available and presents difficulties with the interpretation of the experimental data. Nuclear microprobes can possibly be used as an alternative, but no data are yet available on the use of these instruments for studying zircons.Electron probe microanalysis was used for studying the ore specialization of granitoids as early as in 1977 [11]. Extensive studies of zircons by electron probe microanalysis were reported in works by Krasnobaev [3,9]. In spite of its good locality, electron probe microanalysis exhibits significant limitations in detection limit compared to the bulk analytical methods mentioned above.The main performance characteristics of any analytical method, including electron probe microanalysis, in determining impurity and trace elements is the detection limit, which is closely related to the signal-to-noise ratio, statistical validity, and reliability of the experimental analytical and background signals. The latter condition is substantially related to the change in the effective (average) atomic number of the sample studied, which (along with the overlap of interfering lines and their diffusely scattered tails) is the major factor that affects background intensity upon the electron excitation of x-ray emission. The value substantially depends on the concentration of the heaviest elements in the sample. In zircon, this is impurity of hafnium ( Zr , Hf ) O 2 · SiO 4 ; therefore, the effective atomic number in the absence of the Hf impurity is = 24.84 ; = 62.3. Even a small impurity of hafnium appreciably affects the intensity of the major background component (bremsstrahlung radiation). This effect may be one of the main reasons for the systematic error in measuring (without losing the analyti...

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