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 Current Procedure Other Considerations Precision, Accuracy, and Sources of Error Case Studies and Applications

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%

Half LifeDecay Const Decay ParentDaughter (x 10 9 ) (x )  Th 232 Pb U 238 Pb U 235 Pb

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.

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

Other Approaches… Pioneers of the Technique

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

Montel et al. Point-By Point Approach Others…

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

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

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

Daly Bay, N. Canada

2.0 Ga 1.9 Ga

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

Monazite 5 Large Square, blocky Monazite Analysis (PPM) Calc #YThPbU Age (my) Ave1803 SampStd9.35 N=10 Error of Mean2.96 2sigma5.91