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The isotope potassium-40 (k-40) decays into a fixed ratio of calcium and argon (88.8 percent calcium, 11.2 percent argon).
Since argon is a noble gas, it would have escaped the rock-formation process, and therefore any argon in a rock sample should have been formed as a result of k-40 decay.
For example, in uranium-lead dating, they use rocks containing zircon (Zr Si O Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead.
Zircon has a very high closure temperature, is very chemically inert, and is resistant to mechanical weathering.
With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample.
One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect (too old).
One key assumption is that the initial quantity of the parent element can be determined.The half-life of this process is 1.25 billion years, meaning that it can date significantly older samples.In rubidium-strontium dating a rubidium-87 isotope becomes the daughter product strontium-87.For example, an object with a quarter of its original amount (2x1/2) should be roughly 11,460 years old.In all radiometric procedures there is a specific age range for when a technique can be used.