Uranium–lead dating
Uranium–lead dating uses the decay of uranium-238 and uranium-235 into lead to measure ages of billions of years.
Uranium–lead dating relies on two decay chains. Uranium-238 decays to lead-206 with a half-life of about 4.47 billion years, and uranium-235 decays to lead-207 with a half-life of about 704 million years. Because the same element decays at two different rates, the two clocks can check each other, making the method one of the most reliable in geology.
In meteorites, the related lead–lead method compares lead-207 with lead-206 and is used to date the oldest materials. Calcium-aluminium-rich inclusions give ages of about 4,567 million years, the conventional age of the Solar System. Chondrules, eucrites and angrites have been dated in the same way.
On Earth, uranium–lead dating of zircon crystals has given ages of up to about 4.4 billion years. Recent work on meteorites showed that the ratio of uranium-238 to uranium-235 is not perfectly constant, so modern precise ages measure that ratio for each sample, sharpening the timeline of early Solar System events.
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