1. Absolute Dating
It’s Atomic

2. 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

3. 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

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

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

6. 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

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

8. Decay of 238U to 206Pb

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

10. 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

12. Example

13. 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

14. 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

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

16. 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

17. 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

19. 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

20. 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?