BACKGROUND DETERMINATION AND INSTRUMENTAL PARAMETERS IN ELECTRON MICROPROBE AGE DATING TECHNIQUES

Use of the electron microprobe for age determination (Th-U-total Pb) of monazite, xenotime, zircon, thorite, uraninite and other minerals is enjoying a period of unprecedented interest and growth. Despite the temptations of the relative ease and efficiency of microprobe analysis, accurate results are highly dependent on background acquisition techniques and on careful evaluation of instrumental parameters. Aside from the predictable peak and background interferences, the substitution of unexpected elements, cumulative high order-derived spectral perturbations, and grain boundary fluorescence effects (e.g. K-Ka on U-Mb in K-feldspar or biotite hosted monazite) make routine spectrum acquisition essential. Scanning reveals background interferences and edges, as well as the general curvature, a function of spectrometer efficiency and composition. Background intensities can be extracted directly from scan data using appropriate regression models. If ignored, even minor background interference or shape effects can result in errors of 10s to 100s of m.y. Detailed grain mapping to establish compositional domains is critical in this regard. Although most effort has concentrated on improving Pb analyses, care in actinide analysis must also be emphasized. Measurement of low concentrations of U in high Th materials is complicated by Th interferences and absorption edges. Accuracy also relies on a fundamental understanding of the genesis of the recorded values of counts, current, and time, errors in any one of which can produce significant inaccuracies in concentrations. Refined quantification techniques now produce consistent results in isotopically dated monazite ranging from 300 Ma to 2.7 Ga. Age results should always be evaluated in the context of possible analytical artifacts first in order to avoid applying ad-hoc geochemical or tectonic chimera in complex interpretations. Regular monitoring of instrumental parameters such as current measurement linearity, detector/circuitry dead time, and appropriate energy distribution in counters is essential. Cautiously applied, electron microprobe ages can compliment other geochronologic techniques, potentially offering superior in-situ spatial resolution, as well as efficiency, to the growing arsenal of geochronologic techniques.