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Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites

  • Alain Cocherie EMAIL logo , Eugène Be Mezeme , Olivier Legendre , C. Mark Fanning , Michel Faure and Philippe Rossi
Published/Copyright: March 28, 2015
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American Mineralogist
From the journal American Mineralogist Volume 90 Issue 4

Abstract

High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite.

Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Received: 2004-2-28
Accepted: 2004-10-25
Published Online: 2015-3-28
Published in Print: 2005-4-1

© 2015 by Walter de Gruyter Berlin/Boston

Articles in the same Issue

  1. Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
  2. Contributions to precision and accuracy of monazite microprobe ages
  3. Electron-microprobe age mapping of monazite
  4. Isotopic age constraints from electron microprobe U-Th-Pb dates, using a three-dimensional concordia diagram
  5. Monazite ages in the Chesham Pond Nappe, SW New Hampshire, U.S.A.: Implications for assembly of central New England thrust sheets
  6. Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
  7. 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.)
  8. Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy
  9. Anomalous elastic behavior and high-pressure structural evolution of zeolite levyne
  10. Crystal chemistry of the hydrothermally synthesized Na2(Mn1-xFex 2+)2Fe3+(PO4)3 alluaudite-type solid solution
  11. Thermodynamic properties of uvarovite garnet (Ca3Cr2Si3O12)
  12. Post-aragonite phase transformation in CaCO3 at 40 GPa
  13. Direct formation of the γ-CaSO4 phase in dehydration process of gypsum: In situ FTIR study
  14. The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]
  15. Temperature and composition dependence of structural phase transitions in Ca(TixZr1-x)OGeO4
  16. Low-T neutron powder-diffraction and synchrotron-radiation IR study of synthetic amphibole Na(NaMg)Mg5Si8O22(OH)2
  17. Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope
  18. Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO₂ conditions of natural calc-alkaline magmas
  19. Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex
  20. The true structure of wonesite, an interlayer-deficient trioctahedral sodium mica
  21. Clinoholmquistite discredited: The new amphibole end-member fluoro-sodic-pedrizite
  22. XRMapAnal: A program for analysis of quantitative X-ray maps
  23. Letter. Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps
  24. Letter. Differentiation of commercial vermiculite based on statistical analysis of bulk chemical data: Fingerprinting vermiculite from Libby, Montana U.S.A.
  25. Letter. Crystal structure of single-crystal CaGeO3 tetragonal garnet synthesized at 3 GPa and 1000 °C
  26. Letter. Iron isotope exchange kinetics at the nanoparticulate ferrihydrite surface
  27. Letter. Cation distribution in MgFe2O4 vs. pressure and temperature: Experiments in a piston-cylinder apparatus
https://doi.org/10.2138/am.2005.1303

Articles in the same Issue

  1. Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
  2. Contributions to precision and accuracy of monazite microprobe ages
  3. Electron-microprobe age mapping of monazite
  4. Isotopic age constraints from electron microprobe U-Th-Pb dates, using a three-dimensional concordia diagram
  5. Monazite ages in the Chesham Pond Nappe, SW New Hampshire, U.S.A.: Implications for assembly of central New England thrust sheets
  6. Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites
  7. 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.)
  8. Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy
  9. Anomalous elastic behavior and high-pressure structural evolution of zeolite levyne
  10. Crystal chemistry of the hydrothermally synthesized Na2(Mn1-xFex 2+)2Fe3+(PO4)3 alluaudite-type solid solution
  11. Thermodynamic properties of uvarovite garnet (Ca3Cr2Si3O12)
  12. Post-aragonite phase transformation in CaCO3 at 40 GPa
  13. Direct formation of the γ-CaSO4 phase in dehydration process of gypsum: In situ FTIR study
  14. The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]
  15. Temperature and composition dependence of structural phase transitions in Ca(TixZr1-x)OGeO4
  16. Low-T neutron powder-diffraction and synchrotron-radiation IR study of synthetic amphibole Na(NaMg)Mg5Si8O22(OH)2
  17. Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope
  18. Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching fO₂ conditions of natural calc-alkaline magmas
  19. Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex
  20. The true structure of wonesite, an interlayer-deficient trioctahedral sodium mica
  21. Clinoholmquistite discredited: The new amphibole end-member fluoro-sodic-pedrizite
  22. XRMapAnal: A program for analysis of quantitative X-ray maps
  23. Letter. Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps
  24. Letter. Differentiation of commercial vermiculite based on statistical analysis of bulk chemical data: Fingerprinting vermiculite from Libby, Montana U.S.A.
  25. Letter. Crystal structure of single-crystal CaGeO3 tetragonal garnet synthesized at 3 GPa and 1000 °C
  26. Letter. Iron isotope exchange kinetics at the nanoparticulate ferrihydrite surface
  27. Letter. Cation distribution in MgFe2O4 vs. pressure and temperature: Experiments in a piston-cylinder apparatus
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