The Argon-40 Method of Radioisotope Dating Was Developed in the Geology

question 164

Multiple Choice

The argon-40 method of radioisotope dating was developed in the Geology Department at Stony Brook University to determine the age of rocks brought back from the moon. This method assumes that no argon-40 gas was trapped when the molten rock solidified, and that no argon-40 can escape from the solid rock. One sample that was analyzed had a ratio of potassium-40 to argon-40 of 0.100 (N_K/N_Ar = 0.100) . Potassium-40 decays into argon-40 by positron emission with a half-life of 1.28  10⁹ years. How many years ago was this moon rock formed?

• A)2.6  10 ⁸ years
• B)2.3  10 ⁹ years
• C)4.4  10 ⁹ years
• D)1.3  10 ¹⁰ years
• E)1.3  10 ⁸ years

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