The cracks in zircon crystals can record seismic events and meteorite strikes from our planet’s ancient past. Photo credit: Carles Millan
The oldest confirmed piece of our planet turns out to be a chunk of lovely pink crystal of the mineral zircon, which was found at a sheep station in Western Australia more than a decade ago. Its age, 4.4 billion years, was recently confirmed by Desmond Moser, an associate professor in the Department of Earth Sciences at Western University in London, Ont. Moser’s unique facility, the Zircon and Accessory Phase Laboratory (ZAPLab), hosts a sophisticated electron microscopy array designed specifically to examine these rugged crystals.
“Basically we map tiny twists in the crystal lattice,” says Moser, describing the search for evidence of major planetary events such as the gigantic meteorite that decimated the dinosaurs some 65 million years ago. “We have the ability to detect if there are any reorientations or damage zones in the crystal lattice.”
This technique takes advantage of the fact that zirconium silicate (ZrSiO4) absolutely rejects lead when its crystals form. That makes it the perfect medium for uranium-lead dating, a fundamental approach to determining geological age. The method takes advantage of the fact that two distinct isotopes of uranium — 238U and 235U — have very different half-lives and decay into two distinct isotopes of lead: 206Pb and 207Pb. Uranium has an atomic radius similar to zirconium, so trace amounts of these isotopes make their way into zircon crystals as they form. When those isotopes decay into lead, they become the only lead that will ever be found in the crystal and together make a perfect timekeeper for the age of the crystal. “It’s just amazing the record of events this thing has experienced and preserved for most of Earth’s history,” Moser says.