The most precise method of dating rocks, in which the relative percentages of ‘parent’ and ‘daughter’ isotopes of a given radioactive element are estimated. Early methods relied on uranium and thorium minerals (see uranium–lead dating), but potassium–argon, protactinium–thorium, rhenium–osmium, rubidium–strontium, samarium–neodymium, and carbon-14–carbon-12 are now of considerable importance. Uranium-238 decays to lead-206 with a half-life of 4.5 billion (10⁹) years, rubidium-87 decays to strontium-87 with a half-life of 50.0 billion years, and potassium-40 decays to argon-40 with a half-life of 1.5 billion years. For carbon-14 the half-life is a mere 5730 ± 30 years (see radiocarbon dating), and beyond about 70 000 years the amount of carbon-14 remaining in organic matter is beyond accurate measurement. Compare initial strontium ratio.