How potassium-argon dating works
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by Tas Walker
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.
How can the geologist know? He can’t.
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.
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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.
The scores of dates that have been produced have had a life like hens in a chicken
coop.
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.
Further reading
Related resources
References
- 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.
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Published: 24 June 2008(GMT+10)
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