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"Zircons have produced complicated data that are hard to interpret, though people have pulled dates out," said Mundil, a former UC Berkeley postdoctoral fellow now at the BGC, a non-profit scientific research institute dedicated to perfecting dating techniques for establishing the history of Earth and life on Earth.

"Many of these studies will now have to be redone." The U/Pb isotopic dating technique has been critical in dating geologic events more than 100 million years old, including volcanic eruptions, continental movements and mass extinctions."The beauty of this new technique is that we now can analyze samples we previously could not get an accurate date for," Mundil said.

"This will have a big impact on radio-isotopic dating in general."Mundil and his colleagues, including BGC director Paul Renne, adjunct professor of earth and planetary science at UC Berkeley, used this improved U/Pb technique to establish a more accurate date for the end of the Permian period and the beginning of the Triassic period - 252.6 million years ago, plus or minus 200,000 years.

This boundary coincides with the largest extinction of life on Earth, when most marine invertebrates died out, including the well-known flat, segmented trilobites.

– A new study by geologists at the Berkeley Geochronology Center and the University of California, Berkeley, improves upon a widely used dating technique, opening the possibility of a vastly more accurate time scale for major geologic events in Earth's history.

In a paper published this week in Science, geochemist Roland Mundil of the Berkeley Geochronology Center (BGC) and his colleagues at BGC and UC Berkeley report that uranium/lead (U/Pb) dating can be extremely accurate - to within 250,000 years - but only if the zircons from volcanic ash used in the analysis are specially treated.

Uranium, on the other hand, is so well studied that its decay constant is much better known, making the U/Pb dating technique more accurate, Mundil noted.

U/Pb dating relies upon the decay of naturally occurring uranium and different isotopes of lead."Further application of Mundil's approach will make the geologic time scale more accurate, letting us calibrate extinctions and important events in Earth's history, ranging from 100 million to several billion years ago, with unparalleled accuracy," Renne added.

Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead.

Whereas the U/Pb method yields ages which are more accurate, "Ar/Ar is still king in dating rocks younger than 100 million years and is about as precise as U/Pb methods, though we need to get better data for the decay constants to establish an absolute calibration," Renne said.

"As soon as that calibration is put in place, the Ar/Ar method could become as accurate as U/Pb."The work was supported by the National Science Foundation, the Australian Research Council and the Ann and Gordon Getty Foundation. Ludwig of the BGC and Ian Metcalfe of the University of New England in Armidale, Australia, also participated in the study.

The technique is based on the fact that the naturally occurring isotope potassium-40 decays to argon-40 with a 1.25 billion year half-life.

Comparison of the amount of argon-39 produced in a nuclear reactor to the amount of argon-40 gives a measure of the age of the rocks.

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