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Th M α U M β

CAMECA SX 100: Geochronology and EPMA. - 29082 Monazite Geochronology: an Accurate, Non-Destructive, High Resolution Dating Technique.

20µm Pb M α Y L α Present in a wide variety of rock types, monazite is a robust geochronometer capable of recording multiple crystallization events. Determination of the age of the rocks

Age

Courtesy of UMass Dept. of Geosciences • In-situ dating of monazite by measuring U, Th and Pb allows the linking of microstructure and geochronology data
Rim: 1836 30 MaCenter: 2524 14 Ma + - • New type of information for geologists
439632962198 1009 0(Ma) + • High resolution of complex histories.
- 20µm

Why is Monazite so Suitable for Chemical Dating?

Chemical dating of monazite • No common Pb at time of crystallization: only radiogenic *Pb
UMass Ultrachron:y=1.007 x -8.6712R 2 =0.9999 EPMA age (Ma) Ultrachron Linear (1:1) • Diffusion of Pb, U, Th is negligible • In 100 Ma, Pb is detectable with a microprobe. Measurements of U, Th, Pb allow to obtain ageologically meaningful age.
0200400600TIMS age (Ma) 600 -400 - 200 -0 -

EPMA Dating and Standard Mass Spectrometry Methods

EPMA offers high spatial resolution: with spot size less than 1µm, the lateral resolution is 10 to 100times better than with isotopic methods! Thanks to in situ, non-destructive dating of monazite in a standard polished thin-section, complex growth histories can be resolved within their petrologic context without loss of information. Pre-characterization of monazite for subsequent isotopic analysis. Standard mass spectrometry measurement: EPMA measurement:
Labor intensive Normal thin section Hours required Rapid analysis Destructive methods Minimum damage to the sample Measurements on large areas => average date (5-10µm with SIMS, only 100µm with TIMS & LA-ICPMS) Small area analysis => spatially resolved dating(≈1µm spot, unequalled by any other technique) Age error ≈ problematic due to coarse measurements Age error can be < 10 million years