Earth Through Time
Earth Through Time GEOL 103
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This 4 page Class Notes was uploaded by Jessica Braun MD on Saturday September 12, 2015. The Class Notes belongs to GEOL 103 at West Virginia University taught by Thomas Kammer in Fall. Since its upload, it has received 14 views. For similar materials see /class/202719/geol-103-west-virginia-university in Geology at West Virginia University.
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Date Created: 09/12/15
Geology 103 Outline 6 Page 1 Outline 6 Radiometric Dating Getting dates for absolute time Radiometric Dating Actually a simple technique Only two measurements are needed 1 The parentdaughter ratio measured with a mass spectrometer 2 The decay constant measured by a scintillometer Basis of the Technique Radioactive elements quotdecayquot Decay occurs as an element changes to another element e g uranium to lead The parent element is radioactive the daughter element is stable The decay rate is constant What is Radioactivity Radioactivity occurs when certain elements literally fall apart Usually protons and neutrons are emitted by the nucleus Sometimes an electron is emitted by the nucleus which changes a neutron to a proton Sometimes an electron is captured What causes radioactivity Carbonl4 is produced by cosmic ray bombardment of Nitrogenl4 in the atmosphere All other radioactive elements were produced by supernova explosions before our solar system formed This is called explosive nucleosynthesis Geology 103 Outline 6 Page 2 Common Radioactive Elements Parents and Daughters Carbon14 C14 Nitrogen14 N14 Uranium235 U235 Lead20713b207 Potassium40 K40 Argon40 Ar40 Uranium238 U238 Lead206 Pb206 Rubidium87 Rb87 Strontium87 Sr87 Basis of the Technique As the parent element decays its amount decreases while the amount of the daughter element increases This gives us a ratio of parentdaughter elements The decay rate is geometric rather than linear Unaffected by heat or pressure Key Term HalfLife the amount of time for half the atoms of a radioactive element to decay Doesn t matter how many atoms started half will decay HalfLives Counting halflives Halflives 1 2 3 4 Parent 12 14 18 116 etc Daughter12 34 78 1516 etc Ratio 111317115 Measuring HalfLives Ratios of 1 3 17 115 etc are for whole half lives but any ratios can be measured eg 16 or 142 or 186 Geology 103 Outline 6 Page 3 The Decay Constant 9 The rate of decay is called the decay constant It determines the half life of a radioactive element The decay constant is unique for each radioactive element Measured with a scintillometer The Decay Constant 9 Some values of the decay constant C14 121X104 atoms per year 121 per 10000 U235 972X1010 atoms per year 972 per 10000000000 K40 534X1010 atoms per year 534 per 10000000000 Calculating a Radiometric Date t nat log PDP7 What is the half life of Carbon14 t nat log 111121x10394 t nat log 2121x10394 t 5730 years Some Half Lives Carbon14 5730 years Uranium235 704 MY Potassium40 13 BY Uranium238 45 BY Rubidium87 488 BY Setting the Radiometric Clock When an igneous melt crystallizes parent and daughter elements are chemically separated into different crystals Further radioactive decay keeps the parent and daughter elements in the same crystal Geology 103 Outline 6 Page 4 Setting the Radiometric Clock Individual crystals of the same mineral are dated to give the age of crystallization or cooling Examples include zircon muscovite and biotite Note that whole rock analysis would not give the age of cooling Setting the Radiometric Clock Carbonl4 is different in that it occurs in organic remains rather than in rocks Clock is set when an organism dies Carbonl4 is absorbed by all living organisms from the atmosphere or the food they eat Useful for about 10 half lives or 57000 years Materials dated using the Carbonl4 method Charcoal wood twigs and seeds Bone Marine estuarine and riverine shells Leather Peat Coprolites Soil Pollen Hair Pottery Wall paintings and rock art works Avian eggshell Corals and foraminifera Speleothems Blood residues Textiles and fabrics Paper and parchment Fish remains Insect remains Resins and glues Antler and horn Water Calibrating the Geologic Time Scale Radiometric dates from igneous rocks can be used to indirectly date sedimentary rocks and their fossils Principles such as superposition and intrusive relationships come into play Thousands of radiometric dates have been obtained Age of the Earth 46 BY The oldest rocks found on earth are 40 BY from NW Canada Meteorites and moon rocks are 46 BY Rocks older than 40 BY on earth have apparently been destroyed by weathering and plate tectonics
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