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geol Unit_3_notes geol.

geol Unit_3_notes geol. - Unit 3 Exam 3 Wednesday April...

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Unit 3 Exam 3 Wednesday, April 11 (chapter 7,13,14,2,9,10 & 2 nd part of chapter 8) March 7, 2007 Chapter 8 continued Absolute (radiometric) dating: need some sort of geological clock. Use radioactive materials. o General form: Parent (P-unstable) changes at a fixed rate to Daughter (D-stable) Figure 8.12. If we can measure the amount of P and D in a rock/mineral and the rate of change is known, we can figure out its age. What does the age mean? o For igneous rocks it represents the time when it changes from liquid to solid (example: when the rock solidifies) Examples of P-D pairs used: o Radiocarbon (14carbon) changes (quickly) to nitrogen o An isotope of potassium (K) changes (slowly) to argon (Ar) o Isotopes of Uranium (U) changes (very slowly) to isotopes of lead (Pb) More about rate of change: o Concept of half-life (figure 8.13) when the “clock” starts, there is only (N) atoms of parent, no atoms of daughter. o the time it takes (ex: in years) for half of the P to change to D is called half-life o after 1 half-life (measured in say years), have N/2 atoms of parent only. Missing N/2 (1-(N/2) atoms have changed to daughter. (D/P=1). o After 2 half-lives, have only N/(2x2) (ex: N/4) atoms of parent, and 3N/ 4 atoms of daughter (D/P=3 ) o After 3 half-lives have only N/(2x2x2) (ex: N/8) atoms of parent and 7N/8 atoms of daughter (D/P=7) o Note: at any given time # atoms of P + # atoms of D=N (atoms can neither be created nor destroyed, but can change form) Example: o 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) After 4 half-lives have N/(2x2x2x2) (ex: N/16) of the parent and N-(N/16) i.e. 15N/16 atoms of daughter (D/P=15). So we are dealing with 4 half-lives each of 50 m.y. years rock is 50x4= 200m.y. old March 12, 2007 More on P-D pairs (table 8.1) o For U-Pb , half-life= 4.5 b.y. use method to date fairly old rocks/minerals ~10 m.y. to 4.5 b.y. o For K-Ar , half-life= 1.3 b.y. use this method to date rocks/minerals ~few k.y. to 4.5 b.y. o 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 years. In this case the clock starts when the… (?) o 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. o 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. o Best method: Date ash beds found mixed with sedimentary rocks. These follow the Principle of Superposition Rocks above ash bed are younger. Rocks below ash bed are older.
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