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Professor Bill Compston was interviewed in 2005 for the Interviews with Australian scientists series. By viewing the interviews in this series, or reading the transcripts and extracts, your students can begin to appreciate Australia's contribution to the growth of scientific knowledge.
The following summary of Compston's career sets the context for the extract chosen for these teachers notes. The extract discusses how his research team found the oldest rocks on Earth. Use the focus questions that accompany the extract to promote discussion among your students.
Bill Compston was born in Western Australia in 1931. He began his studies of geology at the University of Western Australia, where he earned a BSc in 1951, a PhD in 1957 and in 1988 received a Doctor of Science. His doctoral research studied carbon isotope ratios in rocks.
He gained a Fulbright scholarship and studied at the California Institute of Technology during 1956. While there he used oxygen isotopes in rocks to measure paleo-temperatures. The isotopic composition in calcite-containing rocks reflects the temperature that was present on Earth at the time the rocks were formed. He returned to the UWA in 1958 as a lecturer in physics where he set up a geochronology (rock-dating) project for whole rocks using isotopes of rubidium and strontium.
In 1961 Compston was appointed to the Department of Geophysics at the Australian National University. He remained at the ANU for the rest of his professional career. His research is distinguished by outstanding contributions in the application of mass spectroscopy to the dating of rocks, particularly using the uranium-lead and rubidium-strontium radioactive decay series.
In 1969 he was the principal investigator for a NASA project dating lunar rocks which were brought back to earth by the Apollo 11 mission.
During the 1970s and 1980s his team at the ANU developed the SHRIMP (Sensitive High Resolution Ion MicroProbe) which revolutionised geochronology with its ability to analyse very small rock samples for lead and uranium without chemical processing. The original SHRIMP and later models are used for a variety of geological applications. In the 1980s Compston's group identified, what was then, the world's oldest known rocks, zircon crystals from Western Australia.
He was elected a Fellow of the Australian Academy of Science in 1971 and received its Mawson medal in 1988 and its Flinders medal in 1998. He became a Fellow of the Royal Society in 1987. In 1995 he received the Clunies Ross Award.
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.
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.
Select activities that are most appropriate for your lesson plan or add your own. You can also encourage students to identify key issues in the preceding extract and devise their own questions or topics for discussion.
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