HR: 0800h
AN: V51A-0332    [Abstracts]
TI: Trace Element Background Modeling for Electron Microprobe Chemical Dating of Monazite
AU: * Loehn, C W
EM: cloehn@vt.edu
AF: Virginia Tech, Department of Geosciences 4044 Derring Hall, Blacksburg, VA 24061, United States
AU: Tracy, R J
EM: rtracy@vt.edu
AF: Virginia Tech, Department of Geosciences 4044 Derring Hall, Blacksburg, VA 24061, United States
AB: Electron probe microanalysis (EPMA) techniques have conventionally been used for major and minor element analysis in geological and material science specimens. EPMA provides both rapid and efficient quantitative measurements of elemental abundance due to the small beam-interaction volume and the ability to vary accelerating voltage and beam current over a wide range allowing for higher count rates to be obtained over small dwell time intervals. The small beam-interaction volume provides the opportunity to obtain multiple analyses within a specimen, limited only by the size of the specimen or grain, providing better statistics. Chemical dating of monazite and xenotime require analysis of the major/minor elements yttrium and thorium, along with trace element analysis for uranium and radiogenic lead. Selection of positions for off-peak background measurements and proper modeling of the continuum can affect both the accuracy and error associated with a single point analysis. An inter-lab study previously performed to correlate monazite isotopic and chemical dating techniques used published positions for major/minor and trace element background measurements, with a linear background fit. Results from that study produced chemical ages that were comparable to ages obtained using ion microprobe (IMP and SHRIMP) U-Pb and Th-Pb isotopic dating techniques, within 2-sigma error. Recent software upgrade to Probe for Windows on the Virginia Tech CAMECA SX50 has provided the ability to visually select positions for background measurements, decreasing the likelihood of inaccurate off-peak background measurements due to spectral interferences. Background modeling of the continuum for trace element measurements of uranium and radiogenic lead can be performed more precisely over a wide continuum range which may require an exponential fit or polynomial fit depending on curvature or over a narrow continuum range that would be associated only with the desired peak area. Effects resulting from wide continuum fit versus narrow continuum fit could produce a significant discrepancy between background-corrected peak intensity using a curved background continuum fit versus that calculated using a linear background continuum fit. This could yield a "reasonable" yet inaccurate geologic age. Previous investigations into the effects of background modeling for trace element analysis have contributed to the need for this detailed study of continuum modeling for EPMA chemical age determination.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3694 Instruments and techniques
DE: 8400 VOLCANOLOGY
DE: 8455 Tephrochronology (1145)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting