Geoscience Reference
In-Depth Information
of applying the potassium-argon dating method to altered basalts. A
newer technique, the argon-argon method, is more precise and has
largely supplanted the older method. Its use narrowed the wide
range of the older age results, leaving a spread that was small but
real, from 67 million years down to 62.5 million years. As more
measurements have been made, this spread has held firm.
To determine whether the Deccan eruptions have the same age
as the K-T boundary, we must know the exact age of that boundary.
So far, I have simply stipulated that the K-T event took place 65 mil-
lion years ago, but without presenting any evidence. How is it that we
date geologic boundaries precisely? As noted earlier, we cannot do so
by using fossils, for they provide only relative ages. We cannot use
the magnetic reversal scale because not only does it give a range of
ages, ultimately it must tie back to radiometric dating using pairs
of parent and daughter atoms, the only method to give absolute
rather than relative ages. In short, to date the K-T boundary pre-
cisely we must find rocks and minerals from that time whose ages
can be measured by one (for comparison ideally several) of the radio-
metric techniques.
The great interest in the Alvarez theory naturally placed the
then-existing estimates of the age of the K-T boundary under close
scrutiny and led to a new set of measurements. Beginning in the
mid-1980s, several precise analyses were made using different
parent-daughter pairs; the results clustered closely around 65.0 mil-
lion years. Thus we can say with rare assurance that the age of the
K-T boundary is 65.0 million years, plus or minus a few hundred
thousand. The K-T is surely the best dated of any of the major
boundaries of the geologic time scale.
I
NDIAN
I
RIDIUM
By the mid-1980s, even the fiercest opponent of the Alvarez theory
had to admit that the iridium anomalies, however much they spread
above and below the K-T boundary and whatever their cause, did
mark the position of the K-T boundary and must reflect a global
event. If Deccan volcanism was the source of that iridium, it might
then be possible to find an iridium-rich K-T boundary layer amidst
the Deccan basalt flows. An intensive search for iridium by the
French team came up empty, however. Finally a group of Indian geol-
ogists, led by N. Bhandari of the Physical Research Laboratory in
Ahmedabad, discovered the iridium needle in the Deccan haystack.
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