Geol. 655 Isotope Geochemistry
Lecture 9
Spring 2003
55
February 6, 2003
G
EOCHRONOLOGY
V
T
HE
UT
H
P
B SYSTEM
: Z
IRCON
D
ATING
Zircon (ZrSiO
4
) is a mineral with a number of properties that make it extremely useful for geochro
nologists.
First of all, it is very hard (hardness 7
1
/
2
), which means it extremely resistant to me
chanical weathering. Second, it is extremely resistant to chemical weathering and metamorphism.
For geochronological purposes, these properties mean it is likely to remain a closed system. Third, i t
concentrates U (and Th to a lesser extent) and excludes Pb, resulting in typically very high
238
U/
204
Pb
ratios.
It is quite possibly na
ture's best clock. Finally, it is
reasonably common as an ac
cessory phase in a variety of
igneous
and
metamorphic
rocks.
The very high
238
U/
204
Pb ra
tios in zircon (and similar
high μ minerals such
as
sphere and apatite) provide
some special geochronological
opportunities and a special
diagram, the
concordia dia
gram
, has been developed to
take advantage of them.
The
discussion that follows can be
applied to any other system
with
extremely
high
238
U/
204
Pb ratios, but in prac
tice, zircons constitute the
principle target for Pb geo
chronologists.
A concordia diagram is sim
ply a plot of
206
Pb*/
238
U vs.
207
Pb*/
235
U.
You should satisfy
yourself that both of these ra
tios are proportional to time.
In essence, the concordia dia
gram is a plot of the
238
U–
206
Pb
age against the
235
U–
207
Pb age.
The ‘concordia’ curve on such a
diagram that is the locus of
points where the
238
U–
206
Pb age
equals the
235
U–
207
Pb age. Such
ages are said to be
concordant
.
Figure 9.2 is an example of a
concordia diagram.
The best way to think about
evolution of Pb/U ratios is to
imagine that the diagram it
self evolves with time, along
with its axes, while the ac
F
igure 1.
Upper.
Separated Zircon crystals. Notice the zoning.
Lower.
Strongly zoned zircon showing differing ages of spots ana
lyzed by ion probe.
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Geol. 655 Isotope Geochemistry
Lecture 9
Spring 2003
56
February 6, 2003
tual data point stays fixed.
Let’s take a 4.0 Ga old zircon as an example.
When it first formed, or
“closed”, it would have plotted at the origin, because had anyone been around to analyze it, they
would have found the
207
Pb*/
235
U and
206
Pb*/
238
U ratios to be 0. Initially,
207
Pb*/
235
U would have in
creased rapidly, while the
206
Pb*/
238
U would have been increasing only slowly.
This is because 4.0 Ga
ago there was a lot of
235
U around (recall that
235
U has a short halflife).
As time passed, the in
crease in
207
Pb*/
235
U would have slowed as the
235
U was ‘used up’. So imagine that the diagram ini
tially 'grows' or 'expands' to the left, expanding downward only slowly.
Had someone been around
3.0 Ga ago to determine 'zircon' ages, he would have drawn it as it appears in Figure 9.3 (of course, he
would have labeled the 3.0 Ga point as 0, the 4.0 Ga point as 1.0, etc.).
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 Fall '10
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 Zircon, concordia diagram

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