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