Geoscience Reference
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the same within the analytical precision as the results from the
other three sites.
In all, Krogh and his colleagues studied 43 K-T zircons. A few
seemed to point to an age of about 418 million years for the parent
rock; several others scatter randomly when plotted on the Concordia
diagram, indicating they have had a more complex history, perhaps
having lost lead twice. But as shown in Figure 15, of the 43 zircons
from all four sites, 30 fall exactly on a straight line (the 418-million-
year-old zircons are omitted from the diagram). These 30 zircons,
found at four sites separated by thousands of kilometers and repre-
senting three completely different geologic settings—a Chicxulub
breccia, Haitian tektites, and K-T boundary clays from two loca-
tions, one 3,500 km from the Yucatan—plot precisely on a single
straight line with a coefficient of correlation, the statistician's test of
"goodness of fit," of 0.998. (When all 43 zircons are included, even
those that obviously have a more complex history, the correlation
coefficient is still a remarkably high 0.985.) If one were to collect
and analyze 30 zircons from a single rock unit, their fit could be no
more perfect. Even though scattered over 3,500 km, these are the
same zircons.
One of the most surprising results of Krogh's work, after one
gets used to the near perfection of the fit, is that so many of the zir-
cons have the same original age. Since we know that the impact that
formed Chicxulub excavated a crater some 20 km deep, a huge slice
of crustal rocks, with diverse ages and compositions, should have
been caught up in the ejecta. Yet most of the zircons give the same
550-million-year upper age. Krogh and his co-workers speculate
that the upper few kilometers at the Yucatan ground zero may have
been made up of zircon-free limestone, so that most of the zircons
came from a single underlying, zircon-bearing layer.
The remarkable sleuthing of Krogh and his colleagues has to
rank as one of the great analytical triumphs of modern geochemistry
(though Officer and Page do not cite them in their 1996 topic or
once mention zircon). Here is what the zircons tell us:
1
. The K-T boundary clay was not formed by volcanism. Had it
been, none of the K-T zircon ages would exceed 65 million
years. Furthermore, volcanic zircons are angular and unshocked,
not the opposite.
2. At least in the Western Hemisphere, the clay had a single
source crater rather than having been derived from multiple
impacts, as some had suggested in the late 1980s.
