Geoscience Reference
In-Depth Information
dating thus provides two essential pieces of information and is one
of the most powerful tools in the geologist's kit.
The ideal substance for uranium-lead dating would be one that
contained no original lead but enough uranium to have produced
measurable amounts of radiogenic lead (lead derived from uranium
decay) over periods of geologic time. It should occur in a variety of
rock types. While we are at it, why not ask for a mineral that is so
hard and chemically inert that it survives weathering, erosion, and
the heat and pressure of metamorphism? Believe it or not, exactly
such a mineral exists: zircon. When zircon crystallizes, it contains
uranium but no lead, thus eliminating the problem of original daugh-
ter atoms. A minor by-product in granitic rocks, zircon sometimes
grows large enough to form a gemstone (a fact well known to view-
ers of home-shopping networks). Because it survives erosion and
every geologic process known except complete remelting, zircon
winds up in a wide variety of rocks and looms much larger in under-
standing earth history than its infrequent occurrence would suggest.
The application of zircon dating to the K-T boundary problem
now begins to become clear. If the rocks that existed at ground zero
contained zircons, which many continental rocks do at least in small
amounts, these zircons might have been shocked, heated, and had
their clocks at least partially reset. The fireball cloud might have
lofted them high and distributed them over thousands of miles. If
suites of such zircons show up in the K-T boundary clay, their
uranium and lead isotopes might have retained both the original age
of the target and the time of the impact—65 million years. They
would then fall along a straight line that intersected Concordia at
65 million years and at some older age defined by the true age of the
target rocks. But surely this is too much too expect.
Bruce Bohor of the U.S. Geological Survey and Tom Krogh, who
now operates one of the world's most sophisticated lead-dating lab-
oratories, at the Royal Ontario Museum in Toronto, examined sam-
ples from the upper K-T layer at the Raton Basin in Colorado and
discovered zircons with the same multiple shock deformation lamel-
lae that characterize impacted quartz. 4 7 Shocked zircon had never
before been seen. Krogh and his Royal Ontario Museum colleague
Sandra Kamo set out to measure the uranium and lead isotopic
ratios of these zircons, but ran into two difficulties. First, although
the analysis had to be done grain by grain, the individual zircons
weighed only from 1 millionth to 3 millionths of a gram and could
not even be seen with the naked eye, making them hard to handle,
to say the least. Second, the zircons contained only between
5 picograms and 200 picograms (a picogram is a trillionth of a gram,
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