The Oldest Rock on Earth

A rock found in the Australian desert nearly 20 years ago has yielded the oldest-known zircon crystal on the earth. In today's issue of Nature, Simon Wilde of Curtin University in Western Australia and his co-workers report that the composition of this ancient crystal offers new ideas about the state of our planet 4.4 billion years ago. Indeed, they found high oxygen isotope ratios and enriched levels of light rare-earth elements caught in the zircons latticeindicating that when it crystallized, the earth had already separated into different kinds of crust. The zircons likeliest parent rock is a granite, formed from magma contaminated by melted continental crust sediments.

Moreover, the isotope and element ratios suggest that this parent rock was created on an earth that had standing wateroceans that may have harbored life. Such conditions are generally thought to have appeared millions of years later, in no small part because the earth was bombarded by meteorites until about 3.9 billion years ago. Based on the new evidence, however, Wilde and his colleagues conjecture that earlier transient oceans may have allowed life to evolve and vanish several times before taking hold.

William Peck, an associate professor at Colgate University, oversaw the collection of oxygen isotope and rare-earth element data, as well as imaging of the crystal (above). "The thing that amazes me the most is that this crystal survived and made it to the surface of the earth," he comments. "This is all the earth has given us, so we've applied all these techniques to eke as much information out of it as we can." He duplicated the results on the same zircon crystal, but because it is the oldest terrestrial rock, there is no way to find corroborative evidence until another one the same age is found. Even then, that rock may reveal a different history, just as rocks around the world today tell different stories about their origins.