GEOLOGY EXAM 3 NOTES BAKSI

GEOLOGY EXAM 3 NOTES BAKSI - Chapter 8 Wednesday, March 07,...

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Chapter 8 Wednesday, March 07, 2007 1:09 PM Absolute (radiometric) dating Need some sort o f geological clock Use radioactive materials General form: Parent (P-unstable) changes at a fixed rate to Daughter (D-stable) Fig 8.12 If we can measure the amount of P & D in a rock/mineral and the rate of change is known, swe can figure out its age. What does the age mean? For igneous rocks represents the time when it changes from liquid to solid (i.e. when the rock solidifies) Examples of P-D pairs used 1. Radiocarbon (14C) changes (quickly) to nitrogen 2. An isotope of potassium (K) changes (slowly) to argon (Ar) 3. Isotopes of uranium (U) change (very slowly) to isotopes of lead (Pb) More about rate of change CONCEPT OF HALF LIFE (Fig 8.13) When "clocl" starts, have only (N) atoms of parent, no atoms of daughter Time (e.g. in years), it takes for half of the P to change into D is called half-life After one half-life (measured in say years), have N/2 atoms of parent only Missing N/2 (N-(N/2)) atoms have changed to daughter (D/P =1) After two half lives, have only N/(2x2) (i.e. N/4) atoms of parents, and 3N/4 atoms of daughter (D/P=3) After 3 half lives have only N/(2x2x2) (i.e. N/8) atoms of parent and 7N/8 atoms of daughter (D/P =7) And so on for more half-lives
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Note: at any given time #atoms of P + # atoms of D=N (atoms can neither be created nor destroyed, but can change form) Example: A mineral contains 10 million atoms of P and 150 million atoms of D. If the half- life of this pair is 50 m.y., what is the age of the rock? (note: D/P = 15) So we are dealing with four half-lives each of 50 m.y. Rock is 50 x 4 = 200 200 m.y. old Add parent and daughter. .divide by parent. 2 to the what power equals that? Multiply by half life Chapter 8 Monday, March 12, 2007 12:41 PM More on P-D pairs Table 8.1 For U-Pb, half-life = 4.5 billion years, use method to date fairly old rocks/minerals, ~10 million years to 4.5 billion years For K-Ar, half-life = 1.3 billion years. Use method to date rocks/minerals ~few k.y. to 4.5 b.y. For radiocarbon, half-life, ~6000 y. Rocks do not contain much carbon. Use this method to date once living material in the range ~0-100,000 y In this case, the block starts when the living material (tree/person) dies Now use these methods to "calibrate" geologic time scale. Problem. Geologic time scale based on sedimentary rocks only, and radiometric ages only possible for igneous rocks Sedimentary rocks cannot be radiometrically dated For sedimentary rock, "age" is when sediment laid down (fossil enclosed). Detrital sedimentary rocks contain broken (solid) grains that have had radioactive clocks ticking in them for a long time. BEST METHOD
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Superposition Rocks above ash bed are ounger Rocks below ash bed are older Figure to be shown in class H G 100 Ma F E Ages: A and B >300 Ma C,D,E,F <300 Ma, >100 Ma G,H <100 Ma D C B A Important Examples of dating U-Pb Method 1. Age of the Earth- 4500 m.y.- based on dating of meteorites formed at the
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GEOLOGY EXAM 3 NOTES BAKSI - Chapter 8 Wednesday, March 07,...

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