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One problem is that potassium is also highly mobile and may move into older rocks.
This depends on the decay of uranium-237 and uranium-238 to isotopes of lead.
To date older fossils, other methods are used, such as potassium-argon or argon-argon dating.
Other forms of dating based on reactive minerals like rubidium or potassium can date older finds including fossils, but have the limitation that it is easy for ions to move into rocks post-formation so that care must be taken to consider geology and other factors.
This method for rock dating is based on the decay of potassium-40 into argon: until the rock solidifies, argon can escape, so it can in theory date the formation of rock.Most rocks contain uranium, allowing uranium-lead and similar methods to date them.Other elements used for dating, such as rubidium, occur in some minerals but not others, restricting usefulness.We can measure directly, for example by using a radiation detector, and obtain a good estimate of by analyzing the chemical composition of the sample.The half-life , specific to each nuclide, can be accurately measured on a pure sample, and is known to be independent of the chemical composition of the sample, temperature and pressure.