One form of uranium–lead dating depends on measuring the ratio of the amount of helium trapped in the rock to the amount of uranium present (since the decay 238U → 206Pb releases eight alpha-particles).
What is the ratio between uranium and lead?
137.7:1 All naturally occurring uranium contains 238U and 235U (in the ratio 137.7:1). Both isotopes are the starting points for complex decay series that eventually produce stable isotopes of lead. U decays to 206Pb (half-life = 4510 Ma, see decay constant) by a process of eight alpha-decay steps and six beta-decay steps.
What does the ratio of uranium and lead in rock tell scientists?
Scientists discovered that rocks could be timepieces -- literally. By measuring the ratio of lead to uranium in a rock sample, its age can be determined. Using this technique, called radiometric dating, scientists are able to see back in time.
How is the uranium to lead ratio in a rock used to estimate its age?
Uranium-Lead Dating Two isotopes of uranium are used for radiometric dating. Uranium-238 decays to form lead-206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life of 704 million years. Uranium-lead dating is usually performed on crystals of the mineral zircon (Figure 11.26).
How old would a sample with 50% uranium-238 and 50% lead-206 be?
After a second half-life, the 50 gm of U-238 would drop to 25 while the 50 gm of Pb-206 would increase to 75. Thus, two half lives. Since the half-life of U-238 is 4.5 Billion years, this rock must be 9 BY old. 1d.
How much uranium 206 is in the rock sample?
Problem #58: A sample of rock contains 64.27 mg of U-238 (isotopic mass 238.0508 amu) and 22.66 mg of Pb-206 (isotopic mass 205.9745 amu). Assuming that radioactive decay of U-238 is the only source of Pb-206, estimate the rocks age. Half life of U-238 radioactive disintegration is = 4.468 x 109 years.
How long does it take for uranium to become lead?
RADIOMETRIC TIME SCALEParent IsotopeStable Daughter ProductCurrently Accepted Half-Life ValuesUranium-238Lead-2064.5 billion yearsUranium-235Lead-207704 million yearsThorium-232Lead-20814.0 billion yearsRubidium-87Strontium-8748.8 billion years2 more rows•13 Jun 2001
How long does it take for 50% of U-238 to decay to PB-206?
about 4.5 billion years After conducting careful measurements on large numbers of U-235 atoms, scientists determined that each U-238 atom has a 50% chance of decaying into Pb-206 during about 4.5 billion years. In other words, the half-life of U-238 is 14 billion years.
What is U-238 and PB-206 used for?
The uranium radioactive series has been used to estimate the age of the oldest rocks in the Earths crust. The ratio of U-238 to Pb-206 in a rock changes slowly as the U-238 in the rock decays.