Deep Time 3 - Chapter 10: Part 3 Deep Time: How Old is Old?...

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Deep Time: How Old is Old? Chapter 10: Part 3
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Radiometric Dating Radiometric dating is aka Geochronology The science of dating geologic events in years Radioactive elements decay at a constant rate that can be measured in the lab and can be specified in years 1950’s: geologist 1 st developed techniques for using measurements of radioactive elements to calculate the ages of rocks
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Brief Chemistry Refresher Atomic Number : number of protons found in an atom’s nucleus Atomic Weight : the total number of protons plus neutrons in an atom’s nucleus Isotope : same atomic number , different atomic weight Ex. Uranium – atomic number 92 238 U – 92 protons, 146 neutrons 235 U – 92 protons, 143 neutrons
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Radioactive Isotopes Isotope : same atomic number , different atomic weight Some isotopes are stable and remain the same forever Radioactive Isotopes are unstable And undergo Radioactive Decay – which converts them into a different element Radioactive Decay occurs through reactions that change the atomic number of the nucleus and form a different element
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Radioactive Isotopes Parent Isotope : the isotope that undergoes decay Daughter Isotope : decay product Examples: Rubidium-87 ( 87 Rb) decays to Strontium-87 ( 87 Sr) Potassium-40 ( 40 K) decays to Argon-40 ( 40 Ar) Uranim-238 ( 238 U) decays to Lead-206 ( 206 Pb)
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http://www.bpc.edu/mathscience/chemistry/history_of_the_periodic_table.html Periodic Table of the Elements
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Radioactive Decay Half-life : how long it takes for ½ of a group of parent isotopes to decay A crystal contains 16 radioactive parent isotopes 1 st half-life: 8 parent isotopes have decayed 2 nd half-life: 4 more parent isotopes decay
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Half –Life of an Isotope
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Radiometric Dating Techniques Radioactive decay proceeds at a known rate and provides a basis for telling time Because an element’s Half-Life is a “constant”, we can calculate the age of a mineral by measuring the ratio of parent to daughter isotopes in the mineral To obtain a radiometric date, must find the right kind of element (radioactive) and the right kind of mineral (containing radioactive elements)
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Parent -> Daughter Half-Life (years) Minerals containing Isotopes 147 Sm -> 143 Nd 106 billion Garnets, micas 87 Rb -> 87 Sr 48.8 billion Potassium-bearing minerals (mica, feldspar, hornblende) 238 U -> 206 Pb 4.5 billion Uranium-bearing minerals (zircon, uraninite) 40 K -> 40 Ar 1.3 billion Potassium-bearing minerals
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Deep Time 3 - Chapter 10: Part 3 Deep Time: How Old is Old?...

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