Geoscience Reference
In-Depth Information
Figure 6.3.
The concordia
curve as defined by
Eq. (6.35). The solid
straight line joining points
t
and
t
'isthe discordia. A
sample that crystallized at
time
t
ago and then lost
lead and closed at time
t
'
ago will have
lead-uranium ratios that
plot on this line. A sample
that plots at point S must
have either lost a fraction
a
/
b
of its lead or gained
uranium in the disturbing
event.
3000 Ma
t
0.5
2000 Ma
a
b
1000 Ma
S
t
'
0
0
10
207
Pb
235
U
the
concordia
curve (Fig. 6.3) because all the points along the curve have con-
cordant U-Pb ages (i.e., the
238
U-
206
Pb age is the same as the
235
U-
207
Pb age).
If a system has been closed to uranium and lead, and the other elements in the
decay scheme, since its formation, and if correction is made for initial lead, then
the
238
U-
206
Pb and
235
U-
207
Pb methods give concordant ages and the isotope
ratios will plot on the concordia. Using this dating method on individual zircon
grains (e.g., using ion-probe techniques) often means that concordant results can
be obtained and the metamorphic history of the rock determined.
In the event of discordant ages, a concordia plot may be able to yield good
estimates both of the ages of the samples and of the time at which the disturbance
occurred. Suppose that crystallization of the samples occurred at time
t
ago and
that at time
t
ago a disturbing event caused the system to lose lead. Since time
t
, the system has been closed. Then, if this disturbing event removed
207
Pb and
206
Pb in the same proportion as that in which they were present in the sample, the
[
206
Pb
235
U]
now
ratios will lie on a straight line called the
discordia
, joining points
t
and
t
on the theoretical concordia curve. The position
of any particular sample on this straight line depends on the amount of lead the
sample lost. Any loss or gain of uranium by the sample during the disturbing
event changes the position of the sample on the line (uranium loss plots on the
extension of the line above the concordia curve to the right of
t
, and uranium
gain plots below to the left of
t
)but
does not
alter the intersection of the line
with the concordia curve and therefore does not alter the values
t
and
t
.Iflead
is lost continuously (e.g., by diffusion) as opposed to loss in a single disturbing
event, the isotope ratios still plot on a straight line. However, in this case, while
t
represents the time of crystallization,
t
is fictitious and has no meaning. Thus, the
stratigraphic and erosional/depositional setting of samples must be known fully
238
U]
now
and [
207
Pb
/
/
