Tiny Time Capsule

Courtesy of the University of Western Ontario

Friday, March 7th, 2008--

Crammed into a 2.8 billion-year-old crystal, no bigger than the diameter of a human hair, is a detailed geologic history of the planet. Despite its age, a zircon crystal--composed of zirconium, silicon and oxygen--is a reliable time capsule, researchers report in the journal Geology.

Zircons grow by incorporating elements around them. The ratio of the element isotopes can date the crystal, and indicate whether the crystal was formed in an ocean or a volcano, for example. But because zircons are so ancient--the oldest known zircon from the Earth was formed 4.4 billion years ago--it wasn't clear whether isotopes diffused out of the crystal. A leaky zircon would make an unreliable recorder.

By comparing zircon data to other data sources and looking at the pattern of oxygen isotope ratios, Desmond Moser, an earth scientist at the University of Western Ontario, and colleagues conclude that zircons do indeed keep their memory. "We've got a new source of the detailed history of the planet that we didn't have confidence in before," says Moser.

The zircons Moser describes in the study grew over the course of 200 million years. They were mined from rocks in the Canadian shield in northern Ontario, Canada where a slice of Earth from thirty kilometers deep rose to the planet's surface when the continents shifted.

Like trees, zircons have growth rings (visible under a microscope). By comparing the difference in oxygen isotopes in the different rings, the researchers can infer how the isotopic ratios in the crystal's growth environment changed over time, says John Valley, co-author on the paper and a geochemist at the University of Wisconsin-Madison. Different isotopic ratios correlate with different geologic features. The isotopes in Moser's zircons suggest that they began life tucked under a volcano in a magma chamber. The next growth rings indicate that the zircons moved to an ocean environment and then finally were sucked deep into the Earth's crust.

Moser is hopeful that as zircon-probing instruments advance, he will be able to squeeze even more information out of zircons. "We don't even understand what most of those rings mean in detail because they're smaller than the resolution of our probes right now," Moser says. For example, the isotope ratios in these zircons indicate that the flat area of Canada where they were found found "was once covered by a mountain range the height of the Rockies or even the Himalayas," Valley says. Higher resolution analysis could reveal how long it took for this mountain range to erode.

Riding on the reputation of zircons as reliable recorders is the hypothesis that the Earth was not a fireball 4.3 billion years ago (as previously thought) but actually covered in liquid oceans. Valley and colleagues published a study suggesting this in 2001--based on analysis of zircons extracted from rock in Australia that was formerly an ancient beach. "When we first published a paper on those zircons it was quite controversial," says Valley.

The next step is to look at extraterrestrial zircons--particularly from Mars, Moser says. "One of the main contributions is that it validates this zircon as a tool for dating the appearance of oceans on rocky planets," Moser says.

--Flora Lichtman