Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
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Michael J. Jercinovic
and
Michael L. Williams
Abstract
Electron probe microanalysis (EPMA) of accessory minerals such as monazite, xenotime, and thorite for minor- and trace-element concentrations and geochronology, requires consideration of beam irradiation effects (increasing heat and charge) as higher current densities and lengthy counting times are employed, and requires careful, detailed assessment of background intensities and interferences. A carbon coat (250 Å thickness) is generally inadequate for prevention of absorbed current fluctuation and beam damage when using the high current densities applied for high precision (e.g., 200 nA, focused beam). Beam irradiation effects include element mobility in monazite, resulting in P loss relative to REE. Coating materials of higher electrical and thermal conductivity are indicated, and use of gold (≥100 Å) is strongly suggested. Systematic compositional and, therefore, age variability can result simply from analytical effects, requiring evaluation of all aspects of data acquisition. The spectra relevant to measurement of Y, Th, Pb, and U are complex, especially in REE-bearing minerals. Acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences, as well as curvature. Background intensities can be extracted directly from scan data by regression. X-ray mapping allows delineation of domains, guiding background acquisition and detailed quantitative analysis. Minor substitution or fluorescence of unexpected elements can compromise analyses, one documented effect being the fluorescence of K in monazite adjacent to, or hosted by, K-feldspar or micas. This effect, clearly evident within 10 micrometers from K-feldspar, can result in erroneous U concentrations leading to misinterpretation of rim .ages. as younger overgrowths. Absorption edges associated with Th also become relevant to the measurement of U at high Th concentrations. Because background intensity is sensitive to variation in average atomic number, backgrounds must be acquired from each identified domain, with particular attention being paid to Th variation. Misapplication of background intensities can result in large age discrepancies, for example, application of backgrounds obtained from a high-Th domain in monazite to measurement of a low-Th domain (1/3 of the amount in the high-Th domain) results in an overestimation of the UMβ background intensity of 0.008 cps/nA, and an overall “increase” in age of 70 m.y.
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
- Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
- Contributions to precision and accuracy of monazite microprobe ages
- Electron-microprobe age mapping of monazite
- Isotopic age constraints from electron microprobe U-Th-Pb dates, using a three-dimensional concordia diagram
- Monazite ages in the Chesham Pond Nappe, SW New Hampshire, U.S.A.: Implications for assembly of central New England thrust sheets
- Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
- Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.)
- Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy
- Anomalous elastic behavior and high-pressure structural evolution of zeolite levyne
- Crystal chemistry of the hydrothermally synthesized Na2(Mn1-xFex 2+)2Fe3+(PO4)3 alluaudite-type solid solution
- Thermodynamic properties of uvarovite garnet (Ca3Cr2Si3O12)
- Post-aragonite phase transformation in CaCO3 at 40 GPa
- Direct formation of the γ-CaSO4 phase in dehydration process of gypsum: In situ FTIR study
- The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]
- Temperature and composition dependence of structural phase transitions in Ca(TixZr1-x)OGeO4
- Low-T neutron powder-diffraction and synchrotron-radiation IR study of synthetic amphibole Na(NaMg)Mg5Si8O22(OH)2
- Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope
- Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO₂ conditions of natural calc-alkaline magmas
- Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex
- The true structure of wonesite, an interlayer-deficient trioctahedral sodium mica
- Clinoholmquistite discredited: The new amphibole end-member fluoro-sodic-pedrizite
- XRMapAnal: A program for analysis of quantitative X-ray maps
- Letter. Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps
- Letter. Differentiation of commercial vermiculite based on statistical analysis of bulk chemical data: Fingerprinting vermiculite from Libby, Montana U.S.A.
- Letter. Crystal structure of single-crystal CaGeO3 tetragonal garnet synthesized at 3 GPa and 1000 °C
- Letter. Iron isotope exchange kinetics at the nanoparticulate ferrihydrite surface
- Letter. Cation distribution in MgFe2O4 vs. pressure and temperature: Experiments in a piston-cylinder apparatus
Articles in the same Issue
- Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
- Contributions to precision and accuracy of monazite microprobe ages
- Electron-microprobe age mapping of monazite
- Isotopic age constraints from electron microprobe U-Th-Pb dates, using a three-dimensional concordia diagram
- Monazite ages in the Chesham Pond Nappe, SW New Hampshire, U.S.A.: Implications for assembly of central New England thrust sheets
- Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
- Comparative isotopic and chemical geochronometry of monazite, with implications for U-Th-Pb dating by electron microprobe: An example from metamorphic rocks of the eastern Wyoming Craton (U.S.A.)
- Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy
- Anomalous elastic behavior and high-pressure structural evolution of zeolite levyne
- Crystal chemistry of the hydrothermally synthesized Na2(Mn1-xFex 2+)2Fe3+(PO4)3 alluaudite-type solid solution
- Thermodynamic properties of uvarovite garnet (Ca3Cr2Si3O12)
- Post-aragonite phase transformation in CaCO3 at 40 GPa
- Direct formation of the γ-CaSO4 phase in dehydration process of gypsum: In situ FTIR study
- The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]
- Temperature and composition dependence of structural phase transitions in Ca(TixZr1-x)OGeO4
- Low-T neutron powder-diffraction and synchrotron-radiation IR study of synthetic amphibole Na(NaMg)Mg5Si8O22(OH)2
- Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope
- Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO₂ conditions of natural calc-alkaline magmas
- Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex
- The true structure of wonesite, an interlayer-deficient trioctahedral sodium mica
- Clinoholmquistite discredited: The new amphibole end-member fluoro-sodic-pedrizite
- XRMapAnal: A program for analysis of quantitative X-ray maps
- Letter. Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps
- Letter. Differentiation of commercial vermiculite based on statistical analysis of bulk chemical data: Fingerprinting vermiculite from Libby, Montana U.S.A.
- Letter. Crystal structure of single-crystal CaGeO3 tetragonal garnet synthesized at 3 GPa and 1000 °C
- Letter. Iron isotope exchange kinetics at the nanoparticulate ferrihydrite surface
- Letter. Cation distribution in MgFe2O4 vs. pressure and temperature: Experiments in a piston-cylinder apparatus
