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Uranium-lead is one of the oldest and most refined radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1- 1 percent range. The method relies on the coupled chronometer provided by the decay of 238U to 206Pb, with a half-life of 4.47 billion years and 235U to 207Pb, with a half-life of 704 million years. This decay occurs through a series of alpha decays, of which 238 U undergoes seven total alpha decays whereas 235U only experiences six alpha decays. 
Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it can be used on other minerals such as monazite, titanite, and baddeleyite. Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Therefore we can assume that the entire lead content of the zircon is radiogenic.
Originally posted by MBF
In my opinion, even black holes will eventually explode spreading material all over the universe to start the cycle again.