How do I calculate the initial Pb isotopic ratios for whole-rock samples with regard to their crystallization ages, which were determined by U-Pb Zircon?
Depending on your situation, the Stacey & Kramers (1975) model can give you theoretical initial U/Pb and Pb/Pb isotopic ratios for a given age. This can be refined with the latest accepted values for the respective decay constants (and their uncertainties). Ken Ludwig's IsoPlot Excel plugin has functions to calculate ratios based on that. Extrapolating the isochrons of your measurements can also give you initial values. However, it becomes much more difficult if you have reason to believe that your rocks have undergone any gain of "common lead" (or Pb loss!)... Even though zircons are a good archive for U (and resulting Pb), fluids migrating through your rock might alter other minerals or form new ones, at another time than the zrn crystallisation age.
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