The hunt for zircons
This will be a more technical piece. I will describe the process of concentrating zircon grains for my undergraduate thesis project alongside with some pictures. Welcome anyone to give it a read, no matter you are interested in geochronology.
Disclaimer: Different labs might do it differently. This is just my own experience. :)
A general overview — How do we know the age of a rock?
It all comes to radioactive decay. Wiki defines radioactive decay as a process of disintegration of an unstable parent nucleus into stable daughter nuclei by emitting radiation.
Zircons (ZrSiO4) exist as accessory minerals in igneous and metamorphic rocks, as detrital grains in sedimentary rocks. They are usually dated by U-Pb techniques because uranium and zirconium have a similar charge of +4 and hence uranium is frequently in substitution for zirconium — but, zircon does not incorporate Pb. Thus, all Pb found in Zircon will be considered radiogenic, i.e.: they are the daughter nuclei from the decay of U. Obtaining the concentration of U and Pb from analytical techniques, we can calculate the age of the formation of the rock. (There is much more about this and I am not going into too much detail here. In this article, I will focus on the sample preparation instead.)
Many basic concepts of physics is applied to separate zircons out of the pile, such as magnetic field strength, specific gravity and density. It might be a fun thing to do — to think about what physics concepts was applied in each step of separation (for physics enthusiasts!).
Step 1: Crushing the rocks
We started off with rock samples collected by the field geologists that weigh about 10–15 lbs. To obtain the zircon minerals, we need to crush them using a jaw crusher, followed by a pulverizer. Contamination can easily happen because the the pulverized rock will be flying everywhere. Hence, throughout the analytical procedures, thorough cleaning the machines, tools, glassware with water and ethanol is a very important process.
Step 2: Shake it off!
The pulverized sample was then placed on the Wilfley shaker table to perform gravity separation. The shaking motion of the Wilfley table creates stratification and separate heavy minerals from light minerals. The heavy minerals is our main interest as that is where zircons will be. To refine the heavy minerals, they were passed again on the Wilfley table. Now, we have three containers, of minerals: the heavy-heavies, the heavies and the light.
After the gravity separation process, a brief magnetic separation was performed to remove metallic remnants from the crushers, and magnetic minerals, such as magnetite from the sample. This step is to reduce the number of times required to pass through the Frantz prevent passing on too many magnetic materials on Frantz magnetic separator so that the quality of separation could be preserved.
The samples were then rinsed with ethanol and dried under the heating lamp before further processing.
Step 3: Sieving
This is to get rid of some of the larger grains that are not pulverized. Uniform grain sizes is important as large grains will get stuck in the Frantz.
Step 4: Free fall separation
This step uses the minerals’ magnetic susceptibility with the help of gravitational fall to remove very magnetic minerals in the sample.
Step 5: the Initial Frantz
The samples will be passed on the Frantz magnetic separator several times before and after the heavy liquids separation. Prior to separation, cleaning the machine carefully using ethanol, ultrasonic cleaner and “air gun” is crucial to prevent cross-contamination between samples. Initial Frantz is an important step of separating the paramagnetic minerals from the non-magnetic minerals. As different minerals have different magnetic susceptibility, the current, forward tilt angle and axial tilt angle were adjusted accordingly until most of the magnetic grains were removed. In simple terms, the feeder will vibrate, sending the samples into the trough in between two magnets. Depending on the magnetic susceptibility of the grains, they will go to a different path via the trough into a different pile.
First, the magnetic field strength was increased by increasing the current from 0.5A, 1.0A to 1.7A. The forward tilt and axial tilt angles were fixed at 5 degrees and 10 degrees respectively. You can find a more detailed description about it here.
Step 6: Heavy Liquids Separation
Second round of gravity separation with the use of heavy liquids. In our lab, we used methylene iodide (MI). Zircons are denser than MI, so it will sink in the heavy liquids, while we get rid of the floats. MI is very toxic and so this process has to be performed in the fume hood. The products from this separation were then rinsed with acetone and dried, before proceeding to the final Frantz separation.
Step 7: the Final Frantz
The heavies obtained from the bottom of the tube were passed through the Frantz at various axial angle (5, 3, 1 and 0 degrees), while fixing the current and the forward slope. Depending on how well the separation and the amount of samples remaining, up to 1 degrees of Frantz separation might be sufficient. The samples were then stored in a dish for hand-picking.
Step 8: Hand-picking Zircons
Picking up a tweezer and I feel like a surgeon working on a huge operation, not on humans but on the minerals! This took me a while to master. All it takes is patience!
Criteria of a high-quality zircon grain: well-faceted aka euhedral, inclusion and bubble-free, colorless and enormous (if possible). I try to pick zircons that are as gemmy-looking as possible!
To prevent zircons from flying everywhere (due to statics), the surface of the dish was wet with ethanol before picking. They were first picked and isolated in a black circle using a tweezer. A DIY pipette was then used to transfer the “finalists” to another dish. The general characteristics of the population were also noted for future reference.
Now, we have completed the mineral separation process and will proceed to annealing and chemical abrasion, which will be written when I’ve done them. :)
I have been enjoying what I am doing in the lab. Everything takes practice to get the hang of it. Although I am performing the same tasks for all of my samples, I am ALWAYS learning new things every time. There will always be a step that I might miss, realize it and correct it when I do this again for another. This is how learning works, right?