How potassium-argon dating works
Published: 24 June 2008 (GMT+10)
One of the most widely used dating methods is the potassium-argon method, which has been applied to ‘dating’ rocks for decades, especially igneous rocks that have solidified from molten magma. The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas. This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid. He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes the initial argon content is zero.1
He assumes that any argon-40 that he measures in his rock sample must have been produced by the radioactive decay of potassium-40 since the time the rock solidified. He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running. And he hopes the rock has remained sealed until the time he collected his sample.
With these assumptions the geologist only needs to measure the relative amounts of potassium-40 and argon-40 in the rock at the present time to be able to calculate an age for the rock. Although it is a simple calculation the big question is whether his assumptions about the rock were correct.
If the rock actually contained some argon-40 when it solidified then the calculated age would be too old. On the other hand, if the rock was later disturbed by a geological upheaval and lost argon the age would be too young. How can the geologist know? He can’t.
What he does is check his calculated age with the ages produced by other dating methods. In other words, he checks to see if his calculated result falls into the range where he expects it to fall, given the geological situation of where he found his rock. He always does this check because no dating method can be trusted on its own.
What happens if the results conflict? It’s simple; the geologist will change his assumed history for that rock.
For example, if the age is higher than he expected he will say that his rock contains ‘excess argon’ or ‘parentless argon’. By this he means that argon gas in his rock has come from the melting of some older rocks deep underground and contaminated his sample with a higher concentration of argon-40, which is why its age is too old.
This is a standard explanation and is essentially a new story about the past, different from the original story that explained how potassium-argon dating works. We could ask ourselves which of the details of this story have been observed.
It is a story about older rocks, melted rocks, solidified rocks and argon gas. It explains what each of these were doing deep inside the earth millions of years ago. The story explains that the behaviour of ‘excess argon’ (it even has a name) made the age too old. Too old compared with what? With the true age of the rock. But wasn’t that what the dating method was supposed to be measuring?
The problem is that although radiogenic argon and excess argon have different names they are exactly the same isotope—argon-40. It is impossible to distinguish between them experimentally. So, how do we work out how much excess argon we have? We can only calculate the amount of excess argon if we know the true age of the rock. But wasn’t that what we were trying to measure?
What happens when the age is too young? In this case the method is again salvaged by changing his assumptions about the past. Often a heating event is invoked to liberate the argon from the solid rock, although other assumptions are made as well.
What happens if the age falls into the range he expected? In this case the geologist assumes that everything went well, and he publishes his result as the crystallization age of the rock.
So although the potassium-argon method has been used for dating rocks for decades, the results it has produced have tended to reinforce the geological framework that already existed. At most it may have modified the framework a little. The scores of dates that have been produced have had a life like hens in a chicken coop. Whenever a new date is introduced it has to find its pecking order within the geological community. Some dates are accepted, some are rejected, some are overturned and some are modified until everything is in its place, and order reigns again.
- Actually, geologists assume that a small amount of argon was present in the rock due to the tiny concentration of argon in the earth’s atmosphere. They usually make a small atmospheric correction for this. Return to text.