Zircon Dating and Earth's Crust Study

We study the rocks of the Earth's crust to work out how the crust formed over time and what implications that has for making sedimentary basins, for having faults and earthquakes. And we've got a rock here that may be an important part of that story. So when we go into the field, we like to take samples of the rocks in order so that we can take them back to the lab and look at them in further detail. So when we do this, we need to make sure that the rock type that we get is nice and fresh and not altered or weathered in any way. And then we write down in our notebooks and on the sample itself we label it and we give it a GPS location and mark it down on the map so we know exactly where we've sampled that rock from. And we can see that it's made up of many, many little minerals. So what we have to do here is we need to look for a mineral called zircon. It's a very small mineral, much smaller than most of these minerals, but it's almost certainly here in very small amounts, has very small crystals. So, the great value of zircon is that it contains uranium, and that uranium decays according to the half-life of the radioactive decay to lead. If we can measure the amount of uranium and the amount of lead, we can work out the age of that rock, the time it crystallised. At the time that this rock formed and the zircon crystals crystallised, this rock was a magma, it was a molten magma deep in the earth, maybe 10 kilometres down, so the dating of this zircon crystal is telling us when the magma crystallized deep in the earth's crust. So this rock has just come straight from the field. We want the zircon grains out of this rock and the way to do that is to crush it. So once we have crushed our rock, we can then separate out the zircons from those heavy minerals by using the FRANS magnetic separator where we pass that mineral fraction. They pass down a chute and they vibrate, they pass across a magnet and the magnetic minerals are separated out on one side and the zircons are separated out on the other side. So once we've separated the zircons from the magnetic minerals, we then use a microscope. So on the screen. here we've got a blown up image of the zircon crystals. On the right here is a piece of pencil lead, so the length here that we can see exposed is about one millimetre. And so you can see just how tiny these little zircon crystals are. But all of them generally have nice well-formed crystal edges and nice pointy tips to them, and these are what we target when we analyse for uranium lead dating. So from here we will pick around... around about 100 zircon crystals individually and we will take those over to our laser ablation machine. We take them over to the mass spectrometer where we zap that with a laser beam and it takes off a little bit of the zircon and then we can measure the amount of uranium and the amount of lead isotopes that's within that zircon and based on the ratio between uranium and lead that we've just analyzed, we can measure the amount of uranium and lead that we can therefore work out the exact age at which that crystal crystallised within a magma. The ratio between the uranium and the lead is all based on the principle of radioactive decay. When that zircon crystallises, it contains 100% uranium and basically no lead. And over a certain period of time, that uranium starts to decay radioactively and it produces lead isotopes, which start increasing. Over time, we gain more and more lead. and less and less uranium. So in a zircon, if we've got lots of uranium and very little lead, we know it's quite a young rock. And if we've got very little uranium and lots of lead, then we know it's relatively a much older rock. Once we've measured our uranium and lead ratio, we can work out basically where we fit on this curve and how old our rock sample is. And for uranium, we've got two isotopes of uranium, uranium-235, which has a half-life of 700 billion years. And then the other uranium isotope that we measure is uranium-238. One half-life, that takes 4.5 billion years. Because this has such a long half-life, we can date really, really old rocks using the uranium-lead method.

So when we do this, we need to make sure that the rock type that we get is nice and fresh and not altered or weathered in any way. And then we write down in our notebooks and on the sample itself we label it and we give it a GPS location and mark it down on the map so we know exactly where we've sampled that rock from. And we can see that it's made up of many, many little minerals. So what we have to do here is we need to look for a mineral called zircon.

It's a very small mineral, much smaller than most of these minerals, but it's almost certainly here in very small amounts, has very small crystals. So, the great value of zircon is that it contains uranium, and that uranium decays according to the half-life of the radioactive decay to lead. If we can measure the amount of uranium and the amount of lead, we can work out the age of that rock, the time it crystallised. At the time that this rock formed and the zircon crystals crystallised, this rock was a magma, it was a molten magma deep in the earth, maybe 10 kilometres down, so the dating of this zircon crystal is telling us when the magma crystallized deep in the earth's crust. So this rock has just come straight from the field.

We want the zircon grains out of this rock and the way to do that is to crush it. So once we have crushed our rock, we can then separate out the zircons from those heavy minerals by using the FRANS magnetic separator where we pass that mineral fraction. They pass down a chute and they vibrate, they pass across a magnet and the magnetic minerals are separated out on one side and the zircons are separated out on the other side.

So once we've separated the zircons from the magnetic minerals, we then use a microscope. So on the screen. here we've got a blown up image of the zircon crystals.

On the right here is a piece of pencil lead, so the length here that we can see exposed is about one millimetre. And so you can see just how tiny these little zircon crystals are. But all of them generally have nice well-formed crystal edges and nice pointy tips to them, and these are what we target when we analyse for uranium lead dating. So from here we will pick around...

around about 100 zircon crystals individually and we will take those over to our laser ablation machine. We take them over to the mass spectrometer where we zap that with a laser beam and it takes off a little bit of the zircon and then we can measure the amount of uranium and the amount of lead isotopes that's within that zircon and based on the ratio between uranium and lead that we've just analyzed, we can measure the amount of uranium and lead that we can therefore work out the exact age at which that crystal crystallised within a magma. The ratio between the uranium and the lead is all based on the principle of radioactive decay.

When that zircon crystallises, it contains 100% uranium and basically no lead. And over a certain period of time, that uranium starts to decay radioactively and it produces lead isotopes, which start increasing. Over time, we gain more and more lead. and less and less uranium.

So in a zircon, if we've got lots of uranium and very little lead, we know it's quite a young rock. And if we've got very little uranium and lots of lead, then we know it's relatively a much older rock. Once we've measured our uranium and lead ratio, we can work out basically where we fit on this curve and how old our rock sample is. And for uranium, we've got two isotopes of uranium, uranium-235, which has a half-life of 700 billion years. And then the other uranium isotope that we measure is uranium-238.

One half-life, that takes 4.5 billion years. Because this has such a long half-life, we can date really, really old rocks using the uranium-lead method.

Transcript for:Zircon Dating and Earth's Crust Study

Transcript for:
Zircon Dating and Earth's Crust Study