Another very interesting mineral is zircon (a silicate of zirconium),
one of the accessory minerals found in small quantities in many
crystalline rocks. Zircon usually occurs in very small grains and is
heavy and hard, so that it can be separated from the other rock without
much difficulty, even though it may take 100 pounds of rock to supply a
gram of zircon.
Zircon usually contains a fair amount of uranium and very little lead.
It holds radiogenically produced lead well, even at relatively high
temperatures. But that is not all. Even if some of the lead is lost,
there is a mathematical way of correcting for it. This technique is
called CONCORDIA ANALYSIS and was developed by G. W. Wetherill, a
physicist then at the Carnegie Institution of Washington. It is based,
again, on the fact that natural uranium has two long-lived isotopes—²³⁸U
and ²³⁵U—and that the lighter one, ²³⁵U, decays faster than the heavier.
The daughter products of both uranium decay processes are isotopes of
the same element, lead—²⁰⁸Pb and ²⁰⁷Pb, respectively. Heavy isotopes are
not separated to any significant degree by chemical processes, so that
if radiogenic lead has been lost from a system for any reason, the other
lead isotopes also will have been lost in whatever proportion they were
present originally.
If we plot a graph of the radiogenic ²⁰⁶Pb/²³⁸U ratio against the
radiogenic ²⁰⁷Pb/²³⁵U ratio for concordant (closed) systems _of all
ages_, we obtain the curved line shown in the figure below. The curve is
the locus of _all_ concordant U-Pb ages and is called _Concordia_. Then
if we test two or more particular zircons of the same age that have lost
different amounts of lead, at about the same time, the plot of their
²⁰⁶Pb/²³⁸U ratios against their ²⁰⁷Pb/²³⁵U ratios will fall on a
straight line that is a chord of the Concordia curve. The upper
intersection of this chord with the curve then will mark the true age of
the zircons. This is an elaborate technique utilizing difficult chemical
procedures, but it has proved invaluable in solving some important
geologic problems.
[Illustration: _The Concordia curve offers a useful way of analyzing
results of age determinations on the mineral zircon._]