Question

Rubidium-strontium dating. The rubidium isotope 8737Rb, a β emitter with a half-life of 4.75×1010yr, is used to determine the age of rocks and fossils. Rocks containing fossils of ancient animals contain a ratio of 8738Sr to 8737Rb of 0.0310.

Assuming that there was no 8738Sr3887Sr present when the rocks were formed, estimate the age of these fossils.

Answer 1

Radioactive decay of into is given below

+

Half life is 4.75 1010 years

Ratio of to found in the fossils of ancient animal = 0.031

Hence fraction of undecayed atoms = 0.969

Hence from radioactive decay equation, we get

where T is half-life period of

ln(0.969) = [ -ln2 t ] / ( 4.75 1010 )

Hence age of rocks and fossil, t = - [ ln(0.969) / ln(2) ] ( 4.75 1010 ) year

t = 2.158 1010 year