The zircon mounts are made, polished, and CL imaged. The next step in the process is to get the chemistry of different growth domains. CL images really come in handy for this. You can see in the images below what is called “sector zoning.” It looks like a little bowtie in the zircon. The brighter areas tend to be U-poor and thus have higher luminescence, whereas the dark areas tend to be U-rich and have lower luminescence.
From the images alone, we can tell that the zircon was growing in a non-equilibrium environment, where the magma composition was changing as the zircon crystal was growing. The growth allowed for some lattice sites to more readily accommodate U, others not.
One way to understand the crystallization and cooling of the magma as the crystal grew is to use the chemistry of the zircon domains. You can think of zircon crystal as a passive recorder in the magma. As it grows, the chemistry reflects all of the changes occurring around it. Using a technique called LA-ICPMS (LA = Laser ablation), a laser is fired at the zircon to liberate little bits of material. Next, the little ablated pieces are put into the ICPMS (inductively coupled plasma mass spectrometer) where they are put in a plasma phase and analyzed for trace elements. Interestingly, titanium concentrations can be used as a proxy for crystallization temperature. The red circles in the image above show 25 micrometer diameter areas that were analyzed.
The video below shows the laser firing at the zircon. Notice the purple spot in the crosshairs, you should see a flash – that’s the laser! The green spots are other spots that are to be analyzed. Notice how each spot is on a different domain in the zircon.
The nice thing about LA-ICPMS is that you can set it and forget it. For example, 600 spots were put down and then left. 360 spots were on zircon from Utah, the remainder on standards. It took about 4.5 hours. BUT!! Once “go” was clicked it ran overnight without any additional effort. It started around 6 PM and was still running by 11 AM. It finished within the next few hours!