Geoscience Reference
In-Depth Information
that you enter a room at exactly 9:00 A.M. and find an hourglass
standing on a table, with three-quarters of its sand in the bottom
cone. You would quickly conclude that the sand has been flowing
for 45 minutes and therefore that the hourglass had been turned
over at 8:15 A.M. Radiometric dating is similar but adds two wrin-
kles. First, imagine that as the grains pass through the constriction
between the upper and lower cones of the hourglass, they change
color, analogous to one element turning into another. Second, imag-
ine that the constriction is adjustable and is tightened a little more
as each grain of sand falls through. The speed with which the grains
fall from top to bottom is then related to how many grains remain
in the upper half: The fewer that are left, the more slowly they fall
through. You could no longer figure out in your head when the
hourglass had been turned over, but if the tightening followed cer-
tain rules, a simple mathematical formula would do the trick.
Of course, in practice radiometric dating is not so simple. Some-
times atoms of the daughter element, inherited from some ancestral
rock, were already present when the decay clock started to run, thus
causing the rock being dated to appear older than it is. In other
cases, parent or daughter atoms are gained or lost after the decay
clock has started to run, throwing off the calculation. Daughter loss
is commonly caused by heat, which expands and opens crystal
structures and allows the loosely bonded daughter atoms to escape
(this is a particular problem with argon atoms when using the older
potassium-argon method). If all the daughter atoms are lost, the
radiometric clock is set back to zero and the time subsequently
measured is not the true, original age of the specimen, but rather the
time that has elapsed since it was heated. However, geochemists
know how to tell when each of these problems has arisen and usu-
ally can correct for them.
The decay of uranium into lead is unique among the geolog-
ically useful parent-daughter pairs because two isotopes of ura-
nium, each with its own half-life, decay into two isotopes of lead:
U 238 decays to Pb 206 and U 235 decays to Pb 207. Thus two
uranium-lead clocks keep time simultaneously. If one plots the
uranium-lead isotopic ratios of samples that have suffered no lead
loss, they lie along a special curve called Concordia (after the god-
dess of agreement), shown in Figure 15. If several samples of the
same rock or mineral plot at the same point on Concordia, we
know that the material has not lost uranium or lead and that the
date obtained is its true original age. Because uranium is present at
measurable levels in a variety of rocks and minerals, the method has
wide applicability.
