The mass ratios of 40Ar to 40K can also be used to date geological materials. Potassium-40 decays by two processes:

a. Why are 40Ar/40K ratios rather than 40Ca/40K ratios used to date materials?
b. What assumptions must be made in using this technique?
c. A sedimentary rock has a 40Ar/40K ratio of 0.95. Calculate the age of the rock.
d. How will the measured age of a rock compare with the actual age if some 40Ar has escaped from the sample?

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ISK +-8e-18Ar (10.7%) t1/2 = 1.27× 109 yr 19k K :Ca +-oe (89.3%)