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

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.

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-1800 - 100’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

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 (1852-1908) Marie Curie (1867-1934) Pierre Curie (1859-1906)

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 (1871-1937) Fredrick Soddy (1877-1956)

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 (1875 -1927) 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

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

Decay of Radioactive Isotopes

Decay of Radioactive Isotopes

Decay of Radioactive Isotopes Half-lives 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.

Decay of Radioactive Isotopes Beta decay of Rb87 to Sr87

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

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

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

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

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

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

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