1. Lecture 9 Absolute Dating of the Earth
Earth History GEOL 2110
Lecture 9
Absolute Dating of the Earth

2. Major Concepts
The discovery of radioactivity in the early 1900’s and the recognition that radioactive decay (a statistical event) occurs at a constant average rate for particular unstable isotopes has provided a means of determining the absolute ages of earth materials
Different radioactive isotopes decay at different characteristic rates, which are portrayed as the half-life of the isotope.
For the radioactive decay of unstable parent isotopes into stable daughter isotopes to provide useful ages requires that the minerals hosting these isotopes remains chemically closed. Weathering, mechanical alteration, or significant reheating can reset the apparent age.

3. Early Ideas about the Age of the Earth
Genesis – 6,000 yrs
Archbishop Usser (1654) 9AM, October 26, 4004 BC
Buffon (1760) – 75,000 yrs
Post-diluvian geologists of the mid ’s of millions
Darwin, 1859 – 300 million based on rates of erosion
“requires unlimited drafts upon the bank of antiquity”
George Scropes (1827, geologist, political economist)
Lord Kelvin (pre-eminent physicist) – 1846 calculated the age of the earth assuming its origin by cooling from a molten state - his estimates ranged from 400 million to 20 million
“The most brilliant argument is no better than its weakest assumption”
Prothero and Dott, p. 98

4. The Discovery of Radioactivity.
1896 – Reported evidence for radioactivity by showing that photographic film became exposed when adjacent to uranium minerals
Came to identify two new radioactive elements – radium and polonium which came to be recognized as intermediate elements formed from the radioactive decay of uranium
All three won the 1903 Nobel Prize in Physics.
Henri Becquerel ( )
Marie Curie
( )
Pierre Curie
( )

5. The Discovery of Radioactivity.
1902 – Rutherford and Soddy recognized that the total amount of radiation emitted from radium was proportional to the number of unstable (radioactive) isotopes present.
They reasoned that the emissions must decrease (decay) in a regular fashion over time - thus was born the idea that radioactive decay could be used as a means of dating minerals.
Ernest Rutherford
( )
Fredrick Soddy
( )

6. The Discovery of Radioactivity
1905 – Boltwood proved that lead (Pb) was the stable (daughter) product of uranium (U) radioactive decay
1907 – Took Rutherford’s suggestion that radioactive decay in uranium-bearing minerals could be used to date the crystallization age of the mineral if the rate of decay was known.
Bertram Boltwood
( )
AGE = Amt of daughter Isotope (Pb) /Amt of parent isotope (U) * decay rate (1010yr)
With the decay of U  Pb being imperceptibly slow and involving intermediate unstable isotopes, he used the relatively fast decay rate of radium. Inaccurate , but OK first order estimate.
Calculate ages ranging from 410 – 2200 Ma for 10 global samples

7. Decay of Radioactive Isotopes
The chemical behavior of an atom is controlled by the number of -electrons, which is the same as the number +protons in order to maintain charge balance. The number of protons in the nucleus (atomic # ) defines the type of element the atom is.
Neutrons have no charge and therefore do not affect the chemical behavior of elements. Neutrons (and protons) do have mass, however, and therefore affect atomic weight of the element

8. Decay of Radioactive Isotopes

9. Decay of Radioactive Isotopes

10. Decay of Radioactive Isotopes
Half-lives
Half-life – the time it takes for half of the original amount of parent isotopes to decay; shows decay to an exponential function
The rate of radioactive decay is a statistical average for the entire population of parent isotopes – gives the probability that a given unstable atom will decay in a given time period.

11. Decay of Radioactive Isotopes
Beta decay of Rb87 to Sr87

12. Isotopic Systems used in Age Dating
Zircon
ZrSiO4
U substitutes for Zr, but Pb does not.

13. Isotopic Systems used in Age Dating
Pb-Pb Age Dating
Th232  Pb208 (14Ga)
U238  Pb206 (4.5Ga)
U235  Pb207 (0.7Ga)
Pb204 is stable, abundance constant
Plotting these three ratios on Pb evolution curves yields precise ages and an internal check on closure

14. Isotopic Systems used in Age Dating
C14 Dating
Half-life – 5,730 yrs
Applications for:
Late Pleistocene and Holocene events
Archeology
Dating organic material

15. Isotopic Systems used in Age Dating
Fission Track Dating
U238 decay involves rare fission of the nucleus rather than alpha decay
Each fission event leave a path of destruction – a track
The density of tracks for a given abundance of U238 is a function of time

16. Resetting the Isotopic Clock
Blocking Temperatures
Different minerals have different temperatures at which they behave as closed systems whereupon they preserve the progression of isotopic ratio evolution with time
Blocking temperatures also vary by isotopic system

17. Resetting the Isotopic Clock
87Rb  87Sr; 86Sr is stable
Rb – chemically substitutes for K
Sr – chemically substitutes for Ca
Isochron
t=2
t=1
87Sr/86Sr
Metamorphism
at t=1
87Sr/86Sr°(m)
t=0
87Sr/86Sr°
Ca-plagioclase
Granite whole rock
K-feldspar
Biotite (K-rich)
87Rb/86Sr

18. Next Lecture
Origin and Early Evolution of the Earth Part 1: Accretion and Differentiation