The
207Pb and 208Pb corrections
The alternative correction methods (i.e. the 207Pb and 208Pb
corrections, Ludwig 2001) work
with two dimensional projections of the 3D concordia. The corrected lead
isotope compositions (or corrected 207Pb/206Pb ages) are
given by the apparent intersections of the relevant projection of the concordia
with the line from the composition of common lead at infinity through the
observed, commonl lead bearing composition.
For zircons which have suffered no lead loss, the 207Pb correction is
equivalent to the discordance correction. Ideally, such zircons are also
concordant in 208Pb/232Th, in which case the 208Pb
correction should give identical results. If, however, the zircon has lost lead,
the 207Pb correction will give ages which are systematically too
low. The magnitude of the bias is a function of the amount of lead lost. For
example, the bias in the 207Pb corrected age of a mid-Proterozoic
zircon which has lost a few percent of its lead, is of an order of magnitude
comparable to that caused by underestimating
t2 by >(>) 500 Ma. Ideally, the 208Pb
correction requires a set of processes which induces a
discordance in 206Pb/238U and 207Pb/235U,
while 208Pb/232Th and 206Pb/238U
remain concordant. Such a scenario may not be very realistic. However, the curvature of the
concordia in the 208Pb/232Th and 206Pb/238U
projection is slight, so that the the projection of a point correced for common
lead by the discordance correctiion may overlap within error with the projection
of the concordia itself . The corresponding 208Pb
corrected age on the 2D 208Pb/232Th and 206Pb/238U
concordia will, however, always be lower than the
upper intercept age t1.
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