PRACTICE EXERCISES ON RADIOMETRIC DATING

Geosciences II Fall 2009

NAME ______KEY_______________________

(Please note that P indicates parent material and D indicates daughter material.)

1) On the chart provided below, fill in the abundances of the parent and daughter material at the various half-lives labeled at the top of the chart.

Number of half-lives	0	1	2	3	4	5	6	7
% Parent Material	100	50	25	12.5	6.25	3.125	1.5625	.78125
% Daughter Material	0	50	75	87.5	93.75	96.875	98.4375	99.21875

2) Using the above chart, estimate the percentage of parent and daughter material that should be present if 5 half-lives have passed. If the length of a half-life is 4 million years, how old is the rock?

3.125% Parent and 96.875% Daughter

Age = 5 half lives x 4MY/half life = 20 Million Years

3) What is the age of a basaltic lava flow which has isotopic abundances of D=87.5%

and P=12.5% and an isotopic half-life of 8 million years?

If D=87.5% and P=12.5%, then 3 half lives have passed.

Age = 3 half lives x 8MY/half life = 24Million Years

4) What is the age of a volcanic ash layer that contains an isotope with an abundance of D=98.4375% and a half-life of 2.5 million years? What percentage of the isotope would remain as parent material?

If D=98.4375%, then 6 half lives have passed.

Age = 6 half lives x 2.5MY/half life = 15 Million Years

P=1.5625%

5) What is the age a granite intrusion which has an isotopic abundance of P=3.125% and a half-life of 18 million years? What percentage of this isotope should be found as daughter material in this rock?

If P=3.125%, then 5 half lives have passed.

Age = 5 half lives x 18MY/half life = 90 Million Years

D=96.875%

6) Two separate isotopes were measured in a rock to determine its age. Isotope A has a half-life of 6.1 million years and 6.25% of the isotope is found as parent material (P=6.25%). Isotope B has a half-life of 2.8 million years and 1.5625% of isotope B is found as parent material. Determine the age based on each isotope. Do these ages agree? Explain.

Isotope A : P=6.25% so 4 half lives have passed.

Age = 4 half lives x 6.1MY/half life = 24.4 Million Years

Isotope B : P=1.5625% so 6 half lives have passed.

Age = 6 half lives x 2.8MY/half life = 16.8 Million Years

The ages do not agree, so one or both must be wrong.

7) A moon-rock sample was dated with 2 separate isotopes, yielding the following results: Isotope A has a half-life of 1.5 billion years and 12.5% of the isotope is found as parent material (P=12.5%). Isotope B has a half-life of 750 million years and 1.5625% of isotope B is found as parent material. Determine the age based on each isotope. Do these ages agree? Explain.

Isotope A : P=12.5% so 3 half lives have passed.

Age = 3 half lives x 1.5BY/half life = 4.5 Billion Years

Isotope B : P=1.5625% so 6 half lives have passed.

Age = 6 half lives x 750MY/half life = 4,500 Million Years = 4.5 Billion Years

The ages do agree and support one another.

8) Why can't accurate age determinations be obtained from clastic (detrital) sedimentary rocks (like conglomerates)? Explain.

Because the radiometric isotopes are reset when the clastic sedimentary rocks are formed. They form at too low of temperatures and pressures. The age of a clastic rock would consist of a composite or average age of the igneous and metamorphic rock fragments that make up the rock.