Long-Lived Radionuclides

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Radioactive isotopes with half lives (t½) exceeding ~500 Ma. They include: 238U (t½ = 4.47 Ga), 232Th (t½ = 14 Ga), 235U (t½ = 0.704 Ga), 40K (t½ = 1.25 Ga), and 87Rb (t½ = 48.8 Ga) and 147Sm (t½ = 106 Ga). Measuring the amount of the parent nuclide and the amount of the daughter isotope, and knowing the decay rate, allows calculation of the time since the daughter isotope began to accumulate. Long-lived radionuclides have reveled the absolute age of the solar system (4.567 Ga) and the timing of major events on the Earth, Moon, and Mars. However, for the parent radionuclide to still be around to measure, it must decay very slowly. This means that the chronometers based on long-lived radionuclides inherently have low precision. The Pb-Pb system, based upon decay of U and Th, provides the most precise dates of early solar system events.