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Earth History GEOL 2110 Lecture 9 Absolute Dating of the Earth

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Major Concepts The discovery of radioactivity in the early 1900s 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.

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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 - 100s 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 (preeminent 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

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The Discovery of Radioactivity. Henri Becquerel (1852-1908) Marie Curie (1867-1934) Pierre Curie (1859-1906) 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.

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The Discovery of Radioactivity. Ernest Rutherford (1871-1937) Fredrick Soddy (1877-1956) 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.

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The Discovery of Radioactivity Bertram Boltwood (1875 -1927) 1905 – Boltwood proved that lead (Pb) was the stable (daughter) product of uranium (U) radioactive decay 1907 – Took Rutherfords 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 (10 10 yr) 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

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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

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Decay of Radioactive Isotopes

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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. Half-lives 01234

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Decay of Radioactive Isotopes Beta decay of Rb 87 to Sr 87

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Isotopic Systems used in Age Dating Zircon ZrSiO 4 U substitutes for Zr, but Pb does not.

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Isotopic Systems used in Age Dating Pb-Pb Age Dating Th 232 Pb 208 (14Ga) U 238 Pb 206 (4.5Ga) U 235 Pb 207 (0.7Ga) Pb 204 is stable, abundance constant Plotting these three ratios on Pb evolution curves yields precise ages and an internal check on closure

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

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Isotopic Systems used in Age Dating Fission Track Dating U 238 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 U 238 is a function of time

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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

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

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

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