Geoscience Reference
In-Depth Information
they resettle, landing on top of younger ones in a way reminiscent of
the upside-down-cake stratigraphy that Shoemaker found at Meteor
Crater.
2
3
Ejecta from Ries Crater contains blocks of all sizes (some
over 1 km) derived from rocks much older than the Miocene age of
the crater. A single Chicxulub drill core could encounter such an
older, out-of-place block on the way down and lead to the erroneous
conclusion that, for that site, the K-T impact crater was falsified.
The first radiometric age report dated the Haitian tektites at
64.5 ± 0.1 million years, and for comparison, a feldspar from the
K-T boundary at Hell Creek, Montana, source of
T. rex
skeletons, at
64.6 ± 0.2 million years.
2
4
These two ages do not quite overlap the
65-million-year age of the K-T boundary within their error bands.
However, such bands reflect only the "intralaboratory" error, that is,
they give the probable range within which the age would fall if mea-
sured again in the same laboratory. But the argon-argon method also
requires reference to an interlaboratory standard, which can intro-
duce small differences when different laboratories analyze the same
sample, enough to bring the Haitian tektite ages into conformity
with the K-T boundary age. Chris Hall of the University of Michi-
gan and colleagues confirmed Izett's results in their own laboratory,
obtaining an age of 64.75 million years for four separate Haitian tek-
tites.
2
5
They noted that the ages of the tektites measured within
their lab agreed so well that they "would make an excellent [argon-
argon] standard"; for an isotope geochemist, this is the ultimate
compliment. Izett's measurement represented the first time that a
K-T impact product, if that is indeed what the Haitian tektites are,
had been directly and absolutely dated.
Next came measurement of the age of the Chicxulub melt rock
itself, and from two different laboratories. First to report were Carl
Swisher and colleagues from the Institute of Human Origins at
Berkeley.
2
6
They measured the age of the Chicxulub igneous rock
and obtained 64.98 ± 0.05 million years, establishing the Chicxulub
event as of exact K-T age. They also dated tektite glass from Haiti
and glass embedded in rocks of K-T age at Arroyo el Mimbral in
northeast Mexico (rocks that Smit and others believe were gener-
ated by the K-T event), and obtained almost exactly 65 million
years for both. A few weeks later, Buck Sharpton of the Lunar and
Planetary Institute and his co-workers reported an argon-argon age
of 65.2 ± 0.4 million years for a different sample of the Chicxulub
melt rock.
2
7
All of these measurements are consistent and show that
the Chicxulub event, the Haitian tektites, and at least one K-T
ejecta deposit, date to precisely 65 million years.
