Origin of radiometric dating
Extrapolating back 4.6 billion years, the ratio just after the earth formed would have been about 0.699.
Rocks derived from recycling of crustal material are derived from sources that have had time to accumulate radiogenic Sr-87, and can have initial ratios of 0.710 or greater.
Fortunately, there is another isotope of strontium around, Strontium 86. Furthermore, since the two isotopes differ by only about 1% in weight, they do not undergo appreciable fractionation from physical processes.
Early attempts at Rubidium-Strontium dating made use of the fact that the Sr-87/Sr-86 ratio in rubidium-poor samples was about 0.7.
The mantle is a lot poorer in Rb than Sr and its Sr-87 content increases slowly.
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Rubidium 87 decays to Strontium 87 with a half life of xx my.
Most scientists and many Christians believe that the radiometric dating methods prove that the earth is 4.5 billion years old.
The textbooks speak of the radiometric dating techniques, and the dates themselves, as factual information.