BACKGROUND ACQUISITION AND SIGNIFICANCE FOR ELECTRON MICROPROBE AGE DATING OF MONAZITE

Electron microprobe geochronology can provide valuable insight into the timing of deformation and metamorphic events. Monazite commonly contains internal compositional domains that can be linked with structural fabrics or metamorphic textures and the electron microprobe offers the spatial resolution necessary to map and analyze these domains, providing critical timing constraints on tectonic history. Recent technique refinements are resulting in improved precision and accuracy, giving a more robust approach to trace element analysis and expanding the potential of microprobe dating. Although all aspects of the methodology must be assessed, background acquisition and evaluation is, perhaps, the largest source of age inaccuracy. Errors due to incorrect background effects can be quite large (10s to 100s of m.y.). Pb is extremely sensitive to background estimation, but U can be at least as critical in some instances. To address this issue, a technique for background acquisition involving high-resolution WDS scanning of spectrometers over relevant background regions was developed. Scan data is displayed, smoothed, then a best-fit line is regressed through selected scan regions using appropriate models. Resulting curves are used to calculate the true background intensity under the peaks of interest. Interferences can be immediately evaluated from scans, and strong absorption edges can be appropriately avoided. Monazite age analysis depends critically on acquisition of appropriate background intensities from each different compositional domain in a sample. Acquisition of trace element peak and background intensities must be done with highly restricted, differential PHA as high order interferences greatly complicate the wavelength spectrum and complicate background measurement. Differential analysis drastically reduces interferences in the background spectra of Th, U, and Pb on PET and should also reduce effects of specular reflection on background intensity at lower wavelengths. Background scanning also shows numerous features which are not generally assumed to be significant or present, such as ThMc (near UMb), KKa (near UMb) and SKa (near PbMa) as well as occasional minor peaks not always identified in wavelength catalogues. A detailed, explicit view of the wavelength spectrum is essential for all trace element analysis.