What Is Common Lead in Lead-Lead Dating?

[Puchinita5]

Homework Statement

Learning about lead-lead dating to determine the age of things (specifically the solar system using meteorites)... I can't seem to understand what the meaning of "common lead" is. So far the best I'm getting is that it is some sort of contaminant.

What is the relationship/difference between the common lead, radiogenic lead that was formed from decay, and initial lead abundances. Is it the same as an initial lead abundance? I'm confused.

Homework Equations

The Attempt at a Solution

 

[SteamKing]

There are several stable isotopes of lead, only one of which is not formed as the result of the radioactive decay of heavier elements. Lead isotopes which result from radioactive decay are called 'radiogenic'. Lead-lead dating uses the ratio of the amount of 'radiogenic' lead to the amount of 'non-radiogenic' lead to estimate age.

See: http://en.wikipedia.org/wiki/Lead-lead_dating

 

[Puchinita5]

yes but in my equations we have COMMON 207, 206, and 204 Pb compared with radiogenic 207 and 206Pb and non-radiogenic 204Pb. So what is common lead? My reference states "common, the lead that was present in the sample at the time of its crystallization and the lead introduced by terrestrial contamination."

So is common lead just the abundance of initial lead combined with contamination lead? Hmm.