ie when rock solidifies Example of P D pair used 1 Radiocarbon 14C changes

# Ie when rock solidifies example of p d pair used 1

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(i.e. when rock solidifies)Example of P – D pair used1.Radiocarbon (14C) changes (quickly) to nitrogen.2.An isotope of potassium (K) changes (slowly) to argon (Ar).3.Isotopes of uranium (U) changes (very slowly) to isotopes of lead (Pb)More about rate of changeConcept of Half-Life(8.13)When “clock” 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 parent, and 3N/4 atoms of daughter (D/P=3)After three 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.Note: At any given time # atoms of P + # atoms of D = N (atoms can neither be created nor destroyed, but can change form).ExamplesA mineral contains 10 million atoms of P and 150 million atoms of D. if the half-life of this pair is 50 million years, what is the age of the rock? (note: D/P 15)After four half-lives have N/(2X2X2X2) (i.e. N/16) of parent and N-(N/16) i.e. 15N/16 atoms of daughter (D/P=15). So we are dealing with four half-lives each 50 million years. Rock is 50 X 4 = 200 million years old.More on P – D pairs (18.1)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.17
For K-Ar, half-life = 1.3 billion years.Use method to date rocks/minerals ~ few K.Y to 4.5 billion years.For radiocarbon, half-life ~ 6000 years.Rocks d 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 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 rocks, “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 MethodDale ash beds mixed with sedimentary rocks. These follow the Principal of Superposition.Rocks above the ash bed are younger.Rocks below the ash bed are older.Figure to be shown in class:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~___________________________________H___________________________________100 M.Y ___________________________________G______________________________________________________________________F___________________________________ E~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(Angular Uniformity)~~~DCBA_____________________________^300 M.Y. Ages:A & B > 300 MYC,D,E,F < 300 MY, > 100 MYG & H < 100 MY18
Important Examples of DatingU-Pb MethodA. Age of the Earth – 4500 million years – based on dating of meteorites formed at the same time as the Earth.B. Explosion of life forms at base of Paleozoic occurred ~ 550 million years ago.C. Faunal extinction even at Paleozoic – Mesozoic boundary occurred ~ 250 million years ago.K—Ar method.1.Faunal extinction event at Mesozoic-Cenozoic boundary occurred ~65 million years ago2.Man-like creatures first appeared in East Africa ~5 million years ago3.

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• Spring '08
• Holmes
• Sedimentary rocks

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