A mineral normally used in cheap jewelry is answering some of the most fundamental questions in the universe at a U of T laboratory.
Established at the ROM during the mid-1970s, and moved to the St. George campus this summer, the Jack Satterly Geochronology Lab is using a revolutionary method for estimating the age of rocks, which in turn can be used to measure milestone events in our universe’s history.
For example, the birth date of the Solar System was recently pinned down to record precision by researcher Yuri Amelin, who estimated the age of the oldest minerals found in meteorites. Researchers used the same technique to understand the formation of supercontinents, and the evolution of the Earth’s crust during our planet’s early history. It was also used to prove conclusively where meteor impacts have occurred. One researcher was even able to connect the largest mass extinction on record with a huge volcanic eruption in Siberia some 250 million years ago. These studies have helped answer some of our deep questions about environmental change and the evolution of life.
The dating technique relies upon the analysis of a trace mineral called zircon, better known for its use in cubic zirconium jewelry. Researchers must use a long and elaborate filtration process in order to extract even a few milligrams of the crystal from 10 kilograms of ground powder. But in spite of how difficult it can be to extract, zircon is the mineral of choice for estimating the age of rocks because it forms a crystal structure that traps single atoms of uranium.
Many elements come in different versions, called isotopes. There are actually two different uranium isotopes inside of a crystal of zircon and, like all radioactive elements, uranium decays over time into a lighter element. The two different uranium isotopes both decay into lead, but at different rates.
Scientists can use the extent to which the uranium has decayed, based on how much lead is present, to estimate how old a rock is. Conventional techniques that don’t use zircon rely on single isotopes. But this lab uses two isotopes, which is much more accurate because the second isotope can be used to verify the estimated age of the first.
However, one of the problems with zircon is its very tight crystal structure. When a uranium atom decays it recoils into the zircon, destroying the integrity of the crystal structure and causing the mineral to lose some of its lead-and therefore making any dating estimates inaccurate.
“When people started analyzing zircon, they thought: ‘Great, we’ve got this fantastic mineral,’ but then they analyzed it and found it had lost a significant amount of its lead for no apparent reason, so they’d always [estimate] ages that were too young,” explains Dr. Don Davis, head of the laboratory. The goal therefore became finding the tiniest, purest particles of zircon.
“The lab has concentrated on refining techniques for looking at smaller and smaller amounts of zircon with the same degree of accuracy…we can actually look at single grains, and date them with the same precision, or even tips of grains.” This is the true power of the lab.
The ability to date single grains is extremely useful for studying rocks from the early days of the planet. A rock that contains zircon that formed three billion years ago deep in the crust, and then melted in a volcano to form a new rock a billion years later, will show two ages in a single grain.
“[Our technique] can be quite powerful because you can look into the prehistory of the rocks,” explains Dr. Davis.
