microprobe monazite geochronology: methods, applications, and challenges for the future. michael l....

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) PO4Common accessory phase in igneous, metamorphic and sedimentary rocks…

U : 10’s of ppm => Several wt%

Half Life Decay Const Decay

Parent Daughter (x 109) (x 1010) Th232 Pb208 13.900 0.49475 6 4

U238 Pb206 4.500 1.55125 8 6

U235 Pb207 0.680 9.84850 7 4

Th232 U

238

U235

Decay Constant

99.28%

0.72%

U238

Th232

Th/U = ~4

Total

232

ThPb exp 232 1 208

+

04.238

U .9928 exp 238 1 206

+

235

U .0072 exp 235 1 207

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 PointApproach

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

# Y Th Pb U Pb Age (my)

1 0 56192 5295 2029 5295 1793.5

2 0 56556 5251 1689 5251 1803

3 0 57696 5343 1646 5343 1806

4 0 58049 5303 1569 5303 1791.6

5 0 57534 5267 1438 5267 1806.6

6 0 58692 5365 1384 5365 1812.4

7 0 57188 5192 1318 5192 1803.7

8 0 64459 6186 2631 6186 1798.6

9 0 67398 6601 3249 6601 1793.5

10 0 67501 6472 2450 6472 1821.6

Ave 1803

SampStd 9.35

N= 10

Error of Mean 2.96

2sigma 5.91