Geoscience Reference
In-Depth Information
FIGURE
I 5 Composite diagram showing a section of Concordia (the
smooth curve) and the position of K-T zircons from Chicxulub, Haiti,
Colorado, and Saskatchewan. The fit of the lead-loss line is nearly perfect. A
few points that appear to have a different history are also shown. [Data from
Krogh and colleagues; recalculated by the author.]
Sometimes, however, the uranium-lead ratios of a suite of
related specimens plot not on Concordia, but on a straight line that
intersects it twice, like a chord to an arc. The mathematics of
uranium-lead decay reveal why: When a rock or set of minerals has
been altered and lead has escaped, samples with different degrees of
lead loss plot along a line that intersects Concordia at two points.
The upper, older intercept of the straight line and Concordia gives
the original age of the rock; the lower, younger intercept gives the
time at which the rock lost lead. When measured ratios plot neither
on Concordia nor along a straight line, we know that the geologic
history of the samples is more complicated—they may have passed
through more than one heating event or they may have lost variable
amounts of both uranium and lead. Such results give little or no use-
ful information, and the geochronologist tries again with different
samples. But when several samples do plot on a straight line that
intersects Concordia twice, we know that the material has passed
through only a single lead loss episode and that we have measured
both its original age and the time at which it lost lead. Uranium-lead
