Earth History GEOL 2110. Lecture 9 Absolute Dating of the Earth. Major Concepts.
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Absolute Dating of the Earth
“requires unlimited drafts upon the bank of antiquity”
George Scropes (1827, geologist, political economist)
“The most brilliant argument is no better than its weakest assumption”
Prothero and Dott, p. 98
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 element formed from the radioactive decay of uranium
All three won the 1903 Nobel Prize in Physics.
Henri Becquerel (1852-1908)
1902 – Rutheford 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.
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.
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 measured 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
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
0 1 2 3 4
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.
Beta decay of Rb87 to Sr87
U substitutes for Zr, but Pb does not.
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
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
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
87Rb 87Sr; 86Sr is stable
Rb – chemically substitutes for K
Sr – chemically substitutes for Ca
Granite whole rock
Origin and Early Evolution of the Earth
Part 1: Accretion and Differentiation