Rb sr dating example
Rb sr dating example - Online sex
These whole-rock ages were approximately correct, but a much more precise mathod is now in use.
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.Rubidium 87 decays to Strontium 87 with a half life of xx my.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. 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.