You want a long half-life for older rocks.
I think it may be argon, but you need to lookup the half-lives of each of them. You can probably google it.
Why is potassium-argon dating most applicable to dating very old rocks?
potassium’s long half-life
potassium’s short half-life
argon’s long half-life
argon’s short half-life
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Now, I think it might be Potassium.. I just saw anothr post with K -40 has a half life of approx. 1.25 billion years.
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