Zircons, the ideal SHRIMP food

As I understand it, the SHRIMP was built to look for zircons. You have mentioned the use of zircons found in ore bodies or rocks as time markers. What is special about zircons for this purpose?

Several things are special. First of all, uranium atoms fit easily into the zircon site in the crystal lattice. They have the same ionic radius as the zirconium, so when the zircon is crystallising, any atoms of uranium that are in the melt will slide into the growing mineral.

In contrast, lead doesn't fit well. It has a different ionic radius and a different charge balance. So the mineral zircon strongly excludes lead. That is a very good feature, because we have to measure the amount of common lead that is in the mineral we are analysing to obtain the radiogenic lead correctly. The ion probe measures the total mass of lead-207, and the total lead-206, but each of those two isotopes starts off with a little bit of common lead-207 and a little bit of common lead-206. The less you have of the common lead the better, and that is why zircon is such a good thing.

Also, zircon is tough physically and is chemically stable, so it doesn't dissolve during low-grade metamorphism and it stands up to being weathered out of an igneous rock and trundled down the rivers into beach sands, where it is incorporated in younger rocks. There are people now analysing the ages of zircons in sedimentary rock to get an idea of the set of rocks that were being weathered, say, 3 billion years ago when a given sandstone was deposited.

What is the importance of being able to age zircons?

Well, this is the way you discover how old rocks are. From studies of zircons by the conventional method, some zircons looked as if they were of multiple ages within a single grain. It was certainly widely recognised that the zircons within rocks called gneisses, many of which were originally sedimentary rocks but had been re-melted, had to be a mixture of old zircons and young zircons formed at the time of reheating. The traditional methods were not suitable for these - they had to use a lot of zircon and so people had to try to identify the new zircons and hand-pick them out from the old ones. This is a very tedious and generally unsuccessful process, so those zircon methods were actually measuring mixtures of ages in minerals and result in age that are neither one thing nor the other.

What we urgently wanted were single, within-grain analyses. We discovered very early on that a single zircon would be a mixture of an older core and a later mantle of younger zircon, perhaps 1000 million years later. But we couldn't tell this in advance. Later another imaging technique, cathodoluminescence, was developed by various people - with electron bombardment you get luminescence excited - and we discovered that different parts of the zircon luminesced differently. These outlined complex growth patterns within single zircons.

So some zircon grains are actually composites of an old core and a younger skin around the edges, and the SHRIMP can pick them out, analyse bits of individual grains and tell you how old those grains are, by the ratio of uranium to lead?

Yes, that's all true. We hadn't realised what a great success the SHRIMP would be for zircons when we built it. And although we did apply it mainly for zircon dating, we also applied it for the study of sulfur isotope ratios in ore bodies and a range of other geological problems.

Finding the oldest piece of Earth

Has the SHRIMP led you to any headline-producing discoveries?

Yes, again as a result of good fortune. Derek Froude, from New Zealand, was doing a PhD with me, and the problem I gave him was to look at all the old sedimentary rocks in the Archean of Australia, get the zircons out of them and find out whether there are any older than about 3.7 billion years. (At the time, those were the oldest known igneous or sedimentary rocks in the world.) So he collected rocks, collaborating with various other geologists and geological surveys who knew the area. And at Mount Narryer, in the Murchison district of West Australia, about 100 kilometres inland from Shark Bay, he hit upon a metasedimentary rock that had plenty of zircons, one of which was about 4.1 billion years old. This was astounding. Also, it commanded world attention, which does a huge amount of good for the lab - and for everyone's ego!

I remember the day when the minerals were analysed. There were several students running the instrument, which we had elected to run more or less on a 24-hour basis because there was so much to be done and because we were still getting it under control. You kept it running unless something went wrong and you had to stop and fix it. You certainly didn't stop and turn it off to have dinner at night; you got someone else to run it.

When Derek saw that the computer output said 4.1 billion he couldn't believe it and he didn't tell anyone at first in case he had done something wrong. So he did it again. This time he got the same answer, and then he found a couple of others and felt confident that this was right. And another student, running the instrument for him for a while, also hit on one of these. This was the big excitement.

This happened in the early '80s, just after the machine really became operational, and it was published in 1983. It had a huge impact - the public as well as scientists all round the world seem to be interested in world records, and this was the oldest mineral fragment found. So it hit the headlines all round the world.

As the oldest piece of Earth?

Yes. We even got into the New York Times when Walter Sullivan, a famous science reporter at the time, wrote an article on it.

Focus questions

• Compston uses radioactive isotopes in geochronology (rock dating). Why is it important to know about the age of rocks?
• Knowing that the SHRIMP uses isotopes of both uranium and lead in its determination of age, why are zircons especially useful for dating rocks?