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.
We've got a rock here that may be an important part of that story.
When we go into the field, we like to take samples of the rocks
and also that we can take the back to the lab and look at them in further detail.
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 have sampled that rock from.
We can see that it's made up of many many little minerals.
We have 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, as very small crystals.
The great value of zircon is it contains uranium.
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 the 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 molten magma deep in the Earth, maybe 10km down.
So the dating of the zircon crystal is telling us when the magma crystallised deep in the Earth's crust.
This rock has just come straight from the field.
We want the zircon grains out of this rock, and the way to do this is to crush it.
Once we have crushed our rock we can separate out the zircons from the heavy minerals
using the the franz magnetic separator where we pass the mineral down a chute and they're vibrated.
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.
Once we've separated the zircons from the magnetic minerals, we then use a microscope.
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.
The length here that we can see exposed is about one millimeter.
You can see just how tiny these zircon crystals are.
All of them generally have nice, well-formed crystal edges
and nice pointy tips to them,
and these are what we target when we analyze for uranium lead dating.
From here we will pick about 100 zircon crystals individually
and we'll take those over to our laser ablation machine.
We take them over to the mass spectrometer where we zap it 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 within that zircon.
Based on the ratio between uranium and lead, that we have just analysed,
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 the zircon crystallizes it contains 100 percent uranium and basically no lead.
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 an 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
where we fit on this curve and how old our rock sample is.
We've got two isotopes of uranium - uranium 235, which has a half-life of 700 million years.
The other uranium isotope that we measure is uranium 238 with a half life of 4.5 billion years.
Because this has such a long half-life, we can date really really old rocks using the uranium lead method.