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