The level of thorium in a sample decreases by a factor of one-half every 4.2 million years. A meteorite is discovered to have only 7.6% of its original thorium remaining. How old is the meteorite?

Question

The level of thorium in a sample decreases by a factor of one-half every 4.2 million years.  A meteorite is discovered to have only 7.6% of its original thorium remaining.  How old is the meteorite?

drwls · Answer

Let N be the number of half-lives in its age (1/2)^N = 0.076 Solve for N and multiply N by 4.2 million years. N = log 0.076 / log 0.5 = 3.72 Age = 15.6 million years