The time when the U (uranium) formed in super
nova process is not relevant. In fact it is taken
into account explicitly when you make an isochron
plot. The idea of radiometric dating is the
Let us say we have a silicate melt and in the
melt there is some Rb (rubidium) (I am going to
use the Rb goes to Sr (strontium) as the example,
since U has three isotopes of Pb (lead), and it
is more complicated, although the principle is the
Now from this liquid a crystal of feldspar
grows and a crystal of biotite also grows. We
want to date the time when these crystals grew in
the magma. This gives us the age of the granite
rock that forms from crystallization of the melt.
This crystal will incorporate some Rb 87 into
its solid lattice. The crystals will incorporate
some Sr 87 from decay of Rb BEFORE incorporation
of Rb into the crystal, as well as some Sr 86
which is non-radiogenic and of course some
radiogenic 87 Rb as well.
Now, if we make a plot of 87/86 Sr measured in
each mineral against the current ratio of
87Rb/86Sr in each mineral , then the plot gives a
straight line, such that the slope is
proportional to the age of crystallization and the
intercept gives the initial 87Sr/86Sr ratio. The
intercept accounts for the radiogenic Sr that was
made ELSE WHERE before the event that gave rise to
the two minerals that ppt from the liquid.
This diagram is called the isochron
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