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

4. Half LifeDecay Const Decay ParentDaughter (x 10 9 ) (x 10 10 )  Th 232 Pb 208 13.9000.4947564 U 238 Pb 206 4.5001.5512586 U 235 Pb 207 0.6809.8485074

5. 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.

6. 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

7. Other Approaches… Pioneers of the Technique

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

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

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

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

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

13. Daly Bay, N. Canada

14. 2.0 Ga 1.9 Ga

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

16. Monazite 5 Large Square, blocky Monazite Analysis (PPM) Calc #YThPbU Age (my) 10561925295202952951793.5 20565565251168952511803 30576965343164653431806 40580495303156953031791.6 50575345267143852671806.6 60586925365138453651812.4 70571885192131851921803.7 80644596186263161861798.6 90673986601324966011793.5 100675016472245064721821.6 Ave1803 SampStd9.35 N=10 Error of Mean2.96 2sigma5.91