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Re: Dating Techniques

As we seem to be talking about dating techniques, perhaps someone could
clear up a query I've had since school:  Presumably the date given (for
dating involving uranium) is the date when the rock was molten.  I can
understand how this affects gaseous elements in the decay series, such as
radon, and maybe elements that gases decay to, but why does melting affect
the ratio of lead to uranium?  The decay series is easy to understand if we
assume we start with a lump of pure uranium, but since the U was made
billions of years ago and has been decaying since then, it becomes much more