Saturday, November 22, 2008

Radioactive Dating: Rubidium-Strontium

Rubidium-Strontium dating:The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87. Strontium-87 is a stable element; it does not undergo further radioactive decay. (Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86.

If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically. (Note that this does not mean that the ratios are the same everywhere on earth. It merely means that the ratios are the same in the particular magma from which the test sample was later taken.) As strontium-87 forms, its ratio to strontium-86 will increase. Strontium-86 is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process. The amount of strontium-86 in a given mineral sample will not change. Therefore the relative amounts of rubidium-87 and strontium-87 can be determined by expressing their ratios to strontium-86: Rb-87/Sr-86 and Sr87/Sr-86 We measure the amounts of rubidium-87 and strontium-87 as ratios to an unchanging content of strontium-86.

Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives. At the same time, the fraction of strontium-87 formed increases from zero and approaches 100% with increasing number of half-lives. The two curves cross each other at half life = 1.00. At this point the fraction of Rb87 = Sr87 = 0.500. At half life = 2.00, Rb87 = 25% and Sr87 = 75%, and so on.

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