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Microprobe Monazite Geochronology:

Microprobe Monazite Geochronology:. Methods, Applications, and Challenges for the Future. Michael L. Williams Michael J. Jercinovic. Course / Workshop Outline. Introduction - General Concepts – Other Approaches Mapping-based Approach Initial Success Problems Analytical Details

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Microprobe Monazite Geochronology:

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  1. Microprobe Monazite Geochronology: Methods, Applications, and Challenges for the Future. Michael L. Williams Michael J. Jercinovic

  2. Course / Workshop Outline • Introduction - General Concepts – Other Approaches • Mapping-based Approach • Initial Success • Problems • Analytical Details • Current Procedure • Other Considerations • Precision, Accuracy, and Sources of Error • Case Studies and Applications

  3. Monazite: (REE, Th) PO4 Common accessory phase in igneous, metamorphic and sedimentary rocks… Rare earths: Major: Ce, La, Nd Minor: As, Si, Ca, Sm Gd, Pr, Dy, … Th : 100’s of ppm => 10’s of wt% U : 10’s of ppm => Several wt% Pb : Very little “common” Pb (ppb => several ppm)

  4. In several hundred million years, total Pb in monazite can accumulate to levels of 100’s of PPM to 1000’s or even wt % levels, … Levels readily measurable by electron microprobe.

  5. In-situ analysis by electron microprobe: • Dates are evaluated in structural and metamorphic context • Multiple age domains within crystals can be evaluated • Age information can be efficiently integrated with petrologic and structural data for numerous samples • Populations of very fine grained monazite can be investigated

  6. Other Approaches… Pioneers of the Technique

  7. Regression – based Approach Cocherie et al. Suzuki et al. Others…

  8. Point-By Point Approach Montel et al. Others…

  9. Mapping-Based Approach • Search mapping to Locate Monazite in textural/structural context • High-resolution mapping to identify internal compositional (age ?) domains

  10. Full-Section Search Mapping Monazite tracing Ce Image Mg Reference map

  11. Full section Mg - Ce map (Migmatite, Grand Canyon)

  12. Daly Bay, N. Canada

  13. 2.0 Ga 1.9 Ga

  14. 99w-19b Mona 15, Neil Bay, Saskatchewan

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