Question

Zircon crystals (ZrSiO4) commonly admit urnaium (U4+) but strongly exclude Pb. Deduce the valence of Pb in magma from this observation and why is this?

Answer 1

Pb can exist in two valence states, Pb2+ and Pb4+.

Parent DecayMode λ Half-life Daughter Ratio
232Th α,β 4.948 x 10-11y-1 1.4 x 1010y 208Pb, 8 4He 208Pb/204Pb, 3He/4He
235U α,β 9.849 x 10-10y-1 7.07 x 108y 207Pb, 7 4He 207Pb/204Pb, 3He/4He
238U α,β 1.551 x 10-10y-1 4.47 x 109y 206Pb, 6 4He 206Pb/204Pb, 3He/4He

During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon lattice.

For more information look at up this link

http://www.geo.cornell.edu/geology/classes/Geo656/656notes13/IsotopeGeochemistry%20Chapter3.pdf