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.

Why zircons?

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.

Rock pieces collected in the field for geochronology analysis.
Left: A jaw crusher; Right: A disk mill, also known as pulverizer.

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.

Wilfley table in action. The pulverized sample was placed on the table top to the right.

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.

Magnetic materials collected after performing gravity separation on the sample.

The samples were then rinsed with ethanol and dried under the heating lamp before further processing.

Left flask was filtering the heavies, while the right flask was filtering the heavy-heavies.
Drying up the heavy-heavies under the heating lamp.

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.

Sieving the heavy-heavies using the №70 mesh.

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.

Magnetic field of 2.7Amp attracted magnetic minerals at the bottom of the vial.

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.

The Frantz in action: classifying the samples into non-magnetic ones (at the back) and magnetic ones (at the front).

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.

Zircons are denser than other felsic minerals (quartz and feldspar) and hence will sink at the bottom of the tube.
How the samples looked after heavy liquids separation under the microscope. This clearly tells the necessity of final Frantz to extract zircon grains.

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.

My precious teeny-weeny bit of samples after the final Frantz.

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.

Left: A general set up of the hand-picking zircons under the microscope. Right: I am supposed to picked the best quality grains into the black circle. (Two circles to learn transporting grains from one place to another for the first time.)
Left: Toolkit for picking zircons (a DIY pipette, a tweezer and a rake). Right: A bed of zircon grains! (Forgive my skills in taking photos through the microscope.)
Zircons that I first picked.
Cataloging samples for archive purposes.

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. :)

Current thought:

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?

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