Radiometric Dating: Rubidium-Strontium
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Rubidium 87 decays to Strontium 87 with a half life of 48.8 b.y. Rubidium and strontium are reasonably abundant and the decay is not branched, unlike potassium-argon. Unfortunately, there is a lot of primordial Strontium 87 around, making it very hard to tell how much was present when the rock formed and how much formed later as a result of radioactive decay.
Fortunately, there is another isotope of strontium around, Strontium 86. Strontium 86 is wholly primordial in origin. 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. Since these samples were poor in rubidium, it could be assumed that the primordial ratio of Sr-87 to Sr-86 was 0.7. Thus, if a sample had 100 Sr-86 atoms total, it could be assumed that 70 of the Sr-87 atoms were primordial. If the sample had 100 atoms of each isotope, we could assume that 70 of the Sr-87 atoms were primordial and 30 were produced by the decay of Rb-87. These whole-rock ages were approximately correct, but a much more precise mathod is now in use.
The strategy now used is to plot an isochron, but to correct for the existence of primordial Sr 87, we plot the ratios (Sr-87)/(Sr-86) versus (Rb-87/Sr-86). Now if the two isotopes of strontium were both primordial, they would plot on a horizontal line, regardless of rubidium content. In samples that contain rubidium, the points will move left and up as the rock ages. The points will move left as the amount of rubidium decreases and up as the amount of Sr-87 increases. The slope of the isochron line gives the age of the rock.
Where the isochron crosses the vertical axis corresponds to a hypothetical sample with no rubidium at all. In other words, the Sr-87/Sr-86 ratio there is the content the rock had before any rubidum decayed. The initial Rb-Sr ratio turns out to be very informative. Rocks derived directly from the mantle have ratios around 0.705 or less. The mantle is a lot poorer in Rb than Sr and its Sr-87 content increases slowly. 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.
Rubidium is directly below potassium in the periodic table, while strontium is directly below calcium. Rubidium has a +1 charge as a cation versus the +2 or strontium, and a larger ionic radius. So the two elements are not entirely chemically compatible. Nevertheless, rubidium does bond ionically, unlike argon. So it is possible for rubidium to diffuse out of place. Nevertheless, rubidium-strontium ages are considerably more robust than potassium-argon ages.
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