When we produced the formula for K-Ar dating, it was reasonable enough to think that there was little to no argon present in the original state of the rock, because argon is an inert gas, does not take part in chemical processes, and so in particular does not take part in mineral formation.
Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in igneous rocks.
100 million years) are determined from the Isochron method applied to Rb/Sr isotopic compositions.
This activity also includes a C-14 option for once-living materials that are less than 50,000 years old. Radioactive Isotopes - the "Clocks in Rocks": Numerical and Relative Ages for Rocks.
In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar method, although the two are not exactly alike. When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium.
We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: as we have noted, rubidium substitutes for potassium, and strontium for calcium.
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