Livermore lab chemist accurately dates first objects to form

by DOE/Lawrence Livermore National Laboratory. Contact: Anne Stark

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LIVERMORE, Calif. -- A geochemist from the Lawrence Livermore National Laboratory, teaming with researchers from the Royal Ontario Museum, the University of Hawaii and Moscow State University, has accurately dated Calcium Aluminum-rich inclusions (CAIs), the oldest objects in our solar system, to be 4.57 billion years old.

In addition, the team has determined that chondrules, another of the earliest objects in the solar system, are 2 to 3 million years younger than CAIs.

The findings, by Ian Hutcheon of LLNL, Yuri Amelin of the Royal Ontario Museum, Alexander Krot of the University of Hawaii and Alexander Ulyanov of Moscow State University, will be published in the Sept. 6, 2002 edition of Science. The article is titled "Pb Isotopic Ages of Chondrules and Ca-Al Rich Inclusions."

Using mass spectrometers to study CAIs and chondrules found in chondritic meteorites, the team was able to determine the ages of the objects by measuring the decay of uranium 238, which is found in both objects and decays into lead. Using an ion microprobe, Hutcheon dated CAIs and chondrules by detecting the decay of aluminum 26 -- also found in both objects -- into magnesium 26.

By comparing the lead and magnesium isotope contents in the CAIs and chondrules, the team determined how old the objects are. Aluminum 26 decays much faster than uranium, and these measurements enabled the team to determine the small difference in age between CAIs and chondrules with unprecedented precision.

"This is the first piece of evidence proving that CAIs are 2 to 3 million years older than chondrules," Hutcheon said. "We were able to make this conclusion based on our measurements." CAIs and chondrules are millimeter-size objects found in primitive meteorites. They formed when dusty regions of the solar nebula were heated to very high temperatures. The dust melted and then crystallized, forming first CAIs and then chondrules. Larger objects, like asteroids and planets, took longer to form and are about 10 to 50 million years younger than the CAIs and chondrules.

"All these objects didn't just form in a snap," Hutcheon said. "By determining the ages of CAIs and chondrules, we can better date asteroids and planets and learn more about the early history of the solar system."

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory, with a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.