Radiometric dating of zircon crystals using an LA-ICP-MS at the UT Austin Geo-Thermochronometry Lab
Laser ablation (LA) -- Inductively coupled plasma (ICP)
-- Mass spectrometer (MS)
3. Laser, which will "ablate" or erode ions off each sample, leaving a tiny pit in the crystal
2. Mirrors to focus the laser down to a few microns in diameter
4. Sample chamber, where thousands of zircon crystals sit in epoxy ready to be "lased"; it also contains known standard samples for comparison
5. Tubing, to carry the ablated crystal material over to the mass spectrometer
1. Computer program, to mark the coordinates of each zircon crystal and tell the laser how long to focus on each sample
7. Mass spectrometer, which measures the concentrations of ions (like U-238) based on their atomic mass
6. Argon gas, which is heated to create the plasma that ionizes the ablated crystal material.
8. Computer program, to calculate the ratio of U-238 to its daughter decay product Pb-206, which tells us how long ago the zircon crystallized from magma
Geologists can determine the ages of rocks and minerals because they "trap" radiometric atoms in their crystal structure, and because radiometric decay is predictable, constant, and measurable. Zircon crystals are used for radiometric dating using the decay of Uranium-238 to Pb-206 (among other decay chains), because zircon contains a lot of uranium and it is also a very hard, durable mineral. Uranium-238 is common and has a long half-life of ~4.4 billion years, so it is useful for measuring old rocks.