U/Pb dates are being collected from the mass spectrometer. Below are some photos of how the samples are loaded for analysis as well as a photo of the analyzing screen.
After the U and Pb are extracted from column chemistry, it gets dried down and moved into the mass spectrometry lab. A couple drops of silica gel are used to re-dissolve the sample and then transfer that with a little spaghetti tube to a rhenium filament. After the liquid dries own on the rhenium, twenty filaments get loaded into a turret and then into the mass spectrometer.
Once in the mass spec, everything is computerized. The temperature of the filaments for ionization of Pb and U is controlled. Pb ionizes at a lower temperature (~1180-1200˚C), so that gets run first. Then the U is run (~1350˚C). The mass spectrometer detectors are basically counting how many times they are hit by ions. In other words, it’s a giant atom counter. Each Pb-U pair takes about 4 hours to run.
Once data is generated, the values are entered into a spreadsheet. The spreadsheet uses the ratios of the isotopes of Pb and the isotopes of U to calculate an age. It also calculates the amount of “common Pb” picograms. Effectively, this is how much Pb is not from radioactive decay, and it’s a measure for how “clean” your lab protocols are. The smaller the number, the better! The spreadsheet also calculates the amount of Pb* in picograms. This is the amount of radiogenic lead that has accumulated from decay of U. In this case, it is better to have lots of radiogenic Pb – it makes it easier to measure. Some of the Black Rock Desert samples only have 40 to 50 FEMTOgrams of lead. That’s tiny! And of course, you want your radiogenic Pb to be much higher than your common Pb.
And this is the real challenge of trying to date young zircon crystals. They are commonly small in size, and there hasn’t been a lot of time to allow for the decay of U into Pb to get a lot of accumulated Pb*. So you end up with really small values that are dependent on how clean your technique is. This sample, however, actually turned out pretty good, but has a spread of dates for the zircon crystals that we analyzed. This is where interpretation now comes into play. What do you think it means to have one rock preserve zircon crystals that have dates ranging from 2.5 to 1.7 Ma?
One of the primary goals of this project is to date the various small volume eruptions in the Black Rock Desert in conjunction with chemical analyses of the lavas to better understand the processes that control magma evolution. To that end, we are using U/Pb dating and trace element geochemistry of zircon crystals to determine both the timing of crystallization and to estimate the temperature of the magma during crystallization. This type of dating is one of the oldest, most well-refined and allows us to date the rocks within 0.1-1% accuracy. This dating technique works by measuring the decay of U to Pb and using its half-life to determine how long the decay process has been going on.
Once the zircon crystals are separated from the volcanic rock, we extract the U and Pb through a technique called column chemistry, shown in the photos below. We dissolve the zircon crystals in Teflon microcapsules, and then load onto micro-columns that are prepared with resin and use a recipe of various acids to slowly extract the U and Pb.
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!
All of the mounts have been made, polished, and the zircon crystals have been imaged using cathodoluminesce (CL)! The photos below show parts of this process.
Polishing: Similar to how we make thin sections, but all the polishing is done by hand with polishing papers that start around 30 micrometers, which is very coarse for grinding, then 15, 9, 3, and 1 micrometers. A link to a video of the polishing process is below.
After polishing, the grains are carbon coated before going into the SEM. There is a newer carbon coater on Westminster’s campus, but the one used for this project at Boise State University is from the 1940s (ok, probably the 70s, but still – old). The sample is loaded into the chamber at the top and sits on the little platform inside. There is a carbon rod that spits out fine carbon dust as you turn up the current.
After the grains get a light coating of carbon, they go into the SEM (the carbon is so that they’re in conductance in the SEM). The SEM has really cool manual controls that adjust the position and the focus of the SEM electron beam. The CL detector is what makes the zircon images. The computer part of this is so that the digital images can be saved, otherwise they just appear on those little screens. CL imaging is one of the coolest parts of the process because you really get to see all the secrets that are trapped within each grain. Here’s a video as well.
The zircon grain mounts were finally set in epoxy! Here are some photos.
You’ll remember from the last update that the zircon crystals are placed on double-sided tape into rows within a 1-cm circle. Once that’s done, a glass ring is placed around the grains and then a plastic ring goes over that. Then the epoxy is poured into the plastic and glass rings until the whole thing is covered.
Once the epoxy sets, the puck is removed from the rings and the zircon grains are embedded within the epoxy. The pucks are about an inch tall and have the diameter about the size of a quarter.
The next photo shows the grains set in the epoxy still within the glass ring. The next step will be to polish down the zircon grains until the center of the grains are exposed. This is why the grains are sized, thicker grains take more polishing than thinner grains. If you have multiple sizes of grains then you might polish away some grains and not polish others enough.
The polishing is all done by hand, without even a polishing turntable to help! The process is similar to creating thin sections. A series of increasingly smaller grit sizes are used to grind away thin layers and polish for a nice reflective surface.
Zircon crystals have been picked to put into an epoxy mount! The photo on the left shows the selected zircon crystals in a petri dish of ethanol. This reduces the static and helps see the high relief of zircon which can give the crystals a little bit of glare. Notice that they are kind of orange-brown and have a variety of shapes and sizes. Mostly they are cute doubly-terminated squat crystals (for this sample).
The grains to be analyzed are picked with a pair of tweezers and transferred to a piece of glass covered with double sided tape using either a pipette, tweezers, or tape. That’s what you’re seeing in the photo on the right, on yellow tape. The zircon grains are then lined up in double rows using tweezers again. Notice the scale of the zircon in comparison to the tweezers in the photo – they’re so small! The circle on the yellow tape is a 1 cm ring. These mounts can be made with multiple samples, each in their own double row, as shown in the third photo. They are sorted by size so that all of the zircon grains that are the same size go onto the same mount, which is important for the polishing step.
Once all of our samples are in the double rows within the 1-cm ring, epoxy is mixed to set the grains in place. After the epoxy cures, the double-sided tape pulls right off! It requires good hand-eye coordination, and a lot of patience!
The images below show the heavy liquids technique we use to separate zircon. We use bromoform which has a density of 2.85 g/cm3. Minerals heavier will sink (like zircon!), lighter stuff will float (like quartz!). After heavy liquids, we are hopefully just left with a bunch of zircon, which you can see in the middle picture. Upon closer inspection under the microscope, you can see that there are pieces of non-zircon material. In the third photo you can see individual grains, which range in length. Some pieces are even dust-sized! The zircon pictured below are from a bulk rock sample (about a gallon) that was collected in the Black Rock Desert. This is actually quite a lot of zircon, another sample that was processed didn’t have nearly as much.
This link will take you to a video of heavy liquids in action!
Now that we have obtained our high-density sample using the water table, it is further separated based on the sample’s magnetic properties. In order to do this, a machine called the “Frantz” is used, which is basically a large magnet. The sample is fed into a feeder that vibrates to funnel the particulates into the machine and down through the magnet; any particles that are attracted to the magnet at that specific magnetic strength will be separated from the particles that are non-magnetic, and each will be fed into separate bins at the end of the feeder line.
The sample is first ran on the Frantz at a low magnetic setting (0.3 A). The split that is non-magnetic is then processed at a higher magnetic setting (1.0A) to purify the sample. This leaves three sample splits: magnetic at low intensity, magnetic at high intensity, and a high intensity non-magnetic split. Zircon can be extracted at magnetic strengths of 0.4 A or higher and have the best yield at magnetic strengths greater than 1.6 A. Thus, the high intensity non-magnetic split will continue on to the heavy liquid step in the zircon extraction process. Come back next week to learn more!
The next step in processing our samples is to separate crystals out by density. Once the rock sample has been crushed and milled to a size of less than one millimeter, we use a water table to extract zircon by density separation. The sample is added to the top of the table while water is poured over it as the table vibrates to agitate the particles and help maintain a steady flow. The table is positioned at an angle, so while the water is moving the sample down the feed, the vibration of the table propels the sample into different groves on the table. Once the sample has reached the end of the table, the different groves filter the sample into three different sample collection containers – low density, medium density, and high density. After the entire sample has run through the table, each sample collection container is emptied into a pan and allowed to dry in an oven. The high-density sample is what we’re interested in, and will continue with the mineral separation process for zircon extraction. The rest of the process will be explained in future blog posts!
Click here to see a video of the water table in action!
The first step to completing our research is, of course, collecting the rock samples from their erupted locations. Once we bring the rocks back to the Westminster College mineral separation lab, we start the crushing process. This allows us to separate the sub-centimeter sized sanidine and sub-millimeter sized zircon crystals from the bulk rock. To do this, we use a couple pieces of equipment. The first is a “jaw crusher” which breaks the rocks down from larger chips to bits that are one centimeter and smaller. We then pour this crushed rock into the disk mill which pulverizes the rock to less than half-millimeter. We sieve this material and then either hand-pick the sanidine crystals for the argon analyses, or keep with the mineral separation process for zircon extraction. That process will be explained in future blog posts!