1100 Ma Absolute Geologic Dating GLG 101 - Physical Geology Bob Leighty.

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Transcript of 1100 Ma Absolute Geologic Dating GLG 101 - Physical Geology Bob Leighty.

  • Absolute Geologic DatingGLG 101 - Physical GeologyBob Leighty

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  • Isotopic dating measures the decay of radioactive isotopes to date rocksDating rocks = Geochronology Absolute Geologic DatingAbsolute Geologic Dating

  • In any element, the number of neutrons can varyThese variations of the same element are called isotopesWhats an Isotope?Absolute Geologic Dating

  • A unstable nucleus (parent) breaks-down (decays) into a new element (daughter)Radioactive DecayAbsolute Geologic Dating

  • The atomic clock starts at this point (parent atoms begin to decay into daughter atoms)As an igneous rock crystallizes, parent atoms get locked into their mineralsRadioactive DecayAbsolute Geologic Dating

  • all parent atomsOver time, parent atoms decay & daughter atoms accumulateRadioactive DecayAbsolute Geologic Dating

  • Which is older & why?Radioactive DecayAbsolute Geologic Dating

  • Radioactive DecayAbsolute Geologic DatingLinear Decay

  • Half-life () = the time it takes for half of the parent atoms to decay into daughter atomsRadioactive DecayAbsolute Geologic DatingExponential Decay

  • After 10 yrs (10) 1 (99%) 1:999Example: for an isotope with a half-life of 1 yearAfter 3 yrs (3) 125 (12.5%) 875 (87.5%) 1:7 After 2 yrs (2) 250 (25%) 750 (75%) 1:3Rock formed1000 (100%) 0 (0%) -Time # of atomsParentDaughterP:D ratioAfter 1 year (1)500 (50%) 500 (50%) 1:1Radioactive DecayAbsolute Geologic Dating

  • Half-lifeAbsolute Geologic Dating

  • Must have a closed system (no parent/daughter atoms added/removed by metamorphism)Crystal formed with only parent isotopes (no daughter atoms in the system at the beginning)> Look for signs of weathering or alterationNeed measurable amounts of parent materialThe rate of decay for various isotopes is constant and can be determinedImportant ConstraintsAbsolute Geologic Dating

  • Need: 1) amount of parent & daughter material, and 2) half-life of the parent isotope2 = 2 * 10 Ma = 20 MaExample An igneous rock has 250 parent atoms & 750 daughter atoms. If the parent isotope = 10 Ma, how old is the rock?Getting a Number AgeAbsolute Geologic Dating # of 012 parent atoms 1000500250

  • yes (e.g., basalt, granite, etc.)> zircon (238U), mica & K-feldspar (40K), or unfossilized organic material (14C) no no (unless its C-rich)Radioactive isotopes (e.g., 238U, 40K, 14C, etc.)Certain mineralsCertain rock typesGetting a Number AgeAbsolute Geologic DatingWhat Do We Need?

  • Interpreting Absolute DatesAbsolute Geologic Dating

  • For sedimentary rocks, the crystallization age of the certain clasts OR if C-rich organic material, when the plant/animal diedInterpreting Absolute DatesAbsolute Geologic Dating

  • For metamorphic rocks: metamorphism alters the atomic clock by changing the amount of parent & daughter atomsThe age of crystallization appears YOUNGER when daughter atoms are lost during metamorphism(e.g., less daughter = less decay = younger)Interpreting Absolute DatesAbsolute Geologic Dating

  • Absolute ages also tell us (indirectly) how old adjacent rocks areAge of Layer ? = 200,000-400,000 a (or 200-400 Ka)Relative & Absolute DatingAbsolute Geologic Dating

  • The Geologic Time Scale is a dual scale based on relative ages of rock sequences with isotopic dates added to itGeologic Time ScaleAbsolute Geologic Dating

  • sill (D) = 100 Ma, dike (F) = 10 Ma, lava flow (H) = 1 MaGeologic Sequence DiagramAbsolute Geologic Dating

  • Deposition of ADeposition of BDeposition of CDeposition of EIntrusion of DTilting & erosionDeposition of GExtrusion of HDeposition of I, J, then KYIntrusion of FO (100 Ma) (10 Ma)(1 Ma)(
  • WWW Links in this Lecture> Absolute dating - http://en.wikipedia.org/wiki/Absolute_dating> Geochronology - http://en.wikipedia.org/wiki/Geochronology> Isotopes - http://en.wikipedia.org/wiki/Isotopes> Element - http://en.wikipedia.org/wiki/Chemical_element > Neutrons - http://en.wikipedia.org/wiki/Neutrons > Protons - http://en.wikipedia.org/wiki/Protons > Nucleus - http://en.wikipedia.org/wiki/Atomic_Nucleus > Radioactive decay - http://en.wikipedia.org/wiki/Radioactive_decay > Exponential decay - http://en.wikipedia.org/wiki/Exponential_decay > Half-life - http://en.wikipedia.org/wiki/Half-life > Geologic time scale - http://www.stratigraphy.org/geowhen/timelinestages.html Absolute Geologic Dating

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