What's Involved in Determining a U-Pb age?

Samples (2-30 kg) are carefully selected in collaboration with field geologists to solve geological problems. In this example, timing of crustal deformation was evaluated from a site northwest of Rankin Inlet, Nunavut.

Rock samples are crushed and heavy mineral grains separated by density and magnetic properties.

Individual mineral grains are characterized, carefully selected, and prepared for analysis using a binocular microscope. Often more detailed characterization using a secondary electron microprobe is warranted.

Selected crystals are carefully washed, weighed and dissolved in acids. Pb and U are separated using ion exchange chromatography under class 100 clean room conditions. Total procedural blanks are < 2 pg (1 e^-12 g) Pb.

The Pb and U are mass analysed using a thermal ionization mass spectrometer to determine their isotopic composition.

Precise Pb/U ratios and the Pb isotopic composition are calculated. As ²³⁸U decays to ²⁰⁶Pb, and ²³⁵U decays to ²⁰⁷Pb at known rates, these ratios are a direct function of the age of the mineral. An age interpretation generally relies on at least three individual analyses depending on the complexity of the sample and the nature of the problem being addressed.