Absolute Dating It’s Atomic

Absolute Dating Relative age Absolute 3rd 12 kya 2nd 1st 1.7 bya Age of an earth material or event in years Relative age Absolute 3rd 12 kya 2nd 1st 1.7 bya youngest 8 kya

Early Earth’s Age Attempts Bishop Ussher Biblical interpretations October 23rd 4004 BC Sedimentation rates 3 my – 500 my Halley/Joly Ocean Salinity 100 my Lord Kelvin Rate of planetary cooling 30 my

Radioactive Revolution Late 19th century Discovered by “accident” Spontaneous transformation of an element into another isotope of the same element Henri Becquerel Marie Curie

Review of Atom Protons – positively charged Neutron – neutral Electron – negatively charged Isotope – element with different number of neutrons (same element different mass)

Two Main Types of Decay Alpha Decay Losses 2 protons and 2 neutrons Mass decreases by 4 Atomic # decreases by 2 Essentially = loss of a positively charged He ion

Beta-minus Decay Neutron splits into proton and electron Atomic number increases by one Mass remains the same

Decay of 238U to 206Pb

Decay of 235U to 207Pb (Half life = 0.704 Ga) Rate controlling decay Image from: Arizona Laserchron Center University of Arizona Dept of Geosciences

How do we know age? Half-life Period of time during which half the parent atoms in a closed system decay to form daughter atoms Spontaneous Unique for each element

Example

Isotope A Isotope B Sample Problem Sample: 1024 grams 1st 512 / 512 2nd 256 / 768 3rd 128 / 896 4th 64 / 960 Isotope A Isotope B Parent Daughter 64 grams 960 grams (10 k) half-life 10,000 20,000 30,000 40,000

Rates of Decay 238U/206Pb 14C/ 14N 3H/3He 4.5 billion 5730 12.3 Parent/Daughter Half Life (yrs) Range (yrs) 238U/206Pb 14C/ 14N 3H/3He 4.5 billion 5730 12.3 10 million to 100 to 70,000 1 to 50

Uses 238U/206Pb 14C/ 14N 3H/3He Parent/Daughter Material Dated Igneous and Metamorphic rocks Organic material Water

The proton stays and the electron is ejected. This changes C into N. Beta decay Neutron splits into 1 proton + 1 electron The proton stays and the electron is ejected. This changes C into N. Carbon Dating

Uranium (U) – Thorium (Th) – Lead (Pb) U-238 decays to Pb-206 Half life = 4.5 billion years U-235 decays to Pb-207 Half life = 713 million years Th-232 decays to Pb-208 Half life = 14.1 billion years

Potassium (K) – Argon (Ar) Potassium is extremely common Half-life is 1.3 billion years Dating range from 100,000 to 4.6 billion years Can be useful for young or old rocks

Practice What is the Parent / Daughter ratio of a sample of radiogenic material if the original sample was 200g. and 3 half-lifes have passed. If the half-life is 20,000 years, how old is the sample? What is the half-life of an isotopic pair if the sample is 2 million years old and contains 625g of parent and 9375g of daughter isotope?