Objective: To know how to calculate half-life and age using radiometric decay constants
Materials: isotope group card, textbook
Instructions: Answer the questions below. If needed, refer to Chapter 2 for information on atoms,
nuclei, etc., and refer to Chapter 3 for information on radiometric dating. You can also watch the
following video on YouTube titled “Types of Decay” by Khan Academy:
1. Define half-life, using the example of eating a pizza.
2. Complete a half-life table by starting at 100% and dividing the % parent abundance by two each
successive half-life. Plot these points on the chart below and draw a smooth curve to connect
Radiometric Dating ELTT
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3. You are an intern at a national laboratory and are tasked with determining the age of biotite in a
granite found in Sierra Nevada. You break open a sample of granite and extract biotite from only
the middle of the specimen. You then use mass spectroscopy to determine the abundances of
parent isotopes of Potassium-40 and daughter isotopes of Argon-40. You determine that roughly
67% of the original parent isotopes remain.
4. Determine the percent abundance of daughter isotopes.
5. Determine the number of half-lives that have passed using your half-life chart above. Be precise!
6. Calculate the age of your sample using the half-life of 1.3 billion years for 40 K.
7. In which Eon (and Era, if listed) is your sample found on the Geologic Time Scale?
8. A lower detectable limit of some isotopes is a parent abundance of about 0.10%, or about 10 half-
lives. Because the passing of each half-life leaves one-half fewer parent atoms, explain why you
cannot use Carbon-14 ( 14 C ) dating to measure specimens from the Mesozoic? Note: the half-life of
14 C is 5,730 (± 30) years.
9. If a bone containing 14 C is 20,000 years old, how many half-lives have passed since the death of the
animal? Remember, age (yrs) = # half-lives × years per half-life.
10. What is the ratio of parent 14 C to daughter 14 N in the bone sample in question 9? Hint: Use your
chart above to determine the percent abundance first, then determine the ratio.
11. What will happen to a specimen of biotite in a granite if the granite is subjected to heat and
pressure enough to recrystallize the biotite? See page 82 if you have Marshak’s textbook.
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