Geoscience Reference
In-Depth Information
or 10"
1
2
gram) of radiogenic lead, almost too little to be measured.
This made analysis especially difficult because, as with iridium, it is
almost impossible to rid a laboratory of the effects of contamination
from environmental lead. To overcome these two problems, Krogh
and Kamo invented new methods of lead analysis, in the process
reducing their laboratory lead background (the amount of environ-
mental lead contamination that cannot be eliminated) to the lowest
of any lab: 2 picograms of lead per experiment.
Using an electron microscope, they found that they could arrange
the tiny Raton Basin zircons visually in a series with unshocked spec-
imens at one end and increasingly shocked and finally granular zir-
cons (whose crystal structures had been completely destroyed) at
the other. On the Concordia diagram, the uranium-lead isotopic
ratios of these zircons plot along a nearly perfect straight line, show-
ing that they came from a single target and experienced a single
episode of lead loss. The line intersects Concordia once at 550 ± 10
million years, which must be the original age of the zircons, and again
at 65.5 ± 3 million years, which must be the time of lead loss. The
more shocked the zircons, the farther down the line they plot, closer
and closer to the lower, 65-million-year intercept. This point is
absolutely critical: Zircons that originated in a volcanic eruption
65 million years ago would have crystallized at that time. They would
possess, and would display, an original age of 65 million years—not
550 million years. Subsequent lead loss would make them appear
younger than 65 million years, not older. The least altered and unal-
tered zircons would give the true age of 65 million years. But just the
opposite is the case for the Raton Basin zircons: The unshocked and
least shocked zircons give the oldest ages, while the most shocked,
including those that are completely shocked, give the youngest,
approaching and in extreme cases reaching 65 million years. Because
65-million-year-old zircons could not produce this result, neither
the zircons nor the clay itself could have come from a 65-million-
year-old volcanic eruption.
Krogh and his group went on to take the next logical step: They
analyzed zircons from both the Chicxulub breccia and from the
Haitian Beloc Formation.
4
8
Plotting the Chicxulub, Haitian, and
Raton Basin zircons on the same Concordia diagram, they found
that 18 of 36 fell on a straight line that intersected Concordia at
545 ± 5 million and 65 ± 3 million years, "as though they had come
from a single sample."
4
9
Finally, the group studied zircons from
the K-T site most distant from Chicxulub, in South Central
Saskatchewan, 3,500 km away.
5
0
They found an upper intercept
age of 548 ± 6 million years and a lower of 59 ± 10 million years,
