Radioactive Dating Geochronology CHM 1020 Sinex

Calculating the age… First-order radioactive decay kinetics P  D P/P o methods – assume P o is known and constant, measure P, ages to 10t 1/2 carbon-14 (t 1/2 = 5730 yr) anthropology or any once living objects new Kr-81 (t 1/2 = 230,000 yr) for ice cores t age = -ln(P/Po)/k

Five dead things – rank age from oldest (1) to youngest (5) Any living thing – modern P o value P/P o P -----> D measure P remaining

Calculating the age… First-order radioactive decay kinetics P  D D/P methods – dating rocks on Earth, Moon, and meteorites, measure D/P by mass spectrometry 206 Pb/ 238 U (t 1/2 = 4.47 billion yr) 207 Pb/ 235 U (t 1/2 = 704 million yr) 208 Pb/ 232 Th (t 1/2 = 14.0 billion yr) 87 Rb/ 87 Sr (t 1/2 = 49.6 billion yr) t age = ln(1+D/P)/k (best for Pb/U in zircons)

P -----> D At time of formation when clock is set D/P ratio by mass spec Five rocks – rank age from oldest (1) to youngest (5) Assume D o = 0

P -----> D At time of formation when clock is set D/P ratio by mass spec What happens if daughter is present when the clock is set? Now you need to develop an isochron plot S = stable reference isotope Now D o > 0