Resetting a clock from Earth's rocks

Science News, August 25, 2001

A refinement in a widely used technique for determining the age of ancient rocks opens up the possibility that Earth may have formed a crust as many as 200 million years earlier than geologists thought.

Scientists can estimate the age of rocks by measuring the proportions of certain radioactive isotopes in them. Most of these isotopes have a half-life--the amount of time it takes for half of the unstable element to decay--between 100,000 and 1 trillion years, says Erik E. Scherer, a geochemist at Munster University in Germany.

One of the isotopes that scientists find most useful for dating some of the oldest rocks is lutetium-176. That form of the rare earth element decays into hafnium-176 and has a half-life of about 37 billion years. The ratio of hafnium-177--a nonradioactive form of that element--to hafnium-176 gives scientists a way to estimate the age of minerals that contain these isotopes. Zircon crystals, which researchers have used to date Earth's crust, are an example, Scherer notes.

Since the early 1980s, scientists had thought that about 194 out of 10 trillion lutetium-176 atoms decay each year. Better measurements have now reduced the annual rate of decay to about 186 atoms out of 10 trillion. That means that rocks scientists have dated using this technique are actually about 4 percent older than previously thought.

The oldest known zircon crystals have a ratio of hafnium-177 to hafnium-176 that's lower than the planet's overall average, Scherer says. Using the previous decay rate for lutetium-176, researchers estimated those rocks to be 4.1 billion years old. If scientists use the updated rate, that age goes up to 4.3 billion years.

In a commentary accompanying the report in the July 27 SCIENCE, Jan Kramers, a geophysicist at Bern University in Switzerland, says that this refinement could help fill in details of the era before Earth's crust first solidified.