Geoscience Reference
In-Depth Information
4.2 Systems with high parent/daughter ratios
There are many important clocks for which the quantities of daughter isotopes at time
t
=
0 can be ignored. The potassium-argon method and uranium-lead method on zircon
are the best known, but the old uranium-helium method has recently been resurrected to
address the chronology of erosion processes. Also worth mentioning in this category are
the many clocks based on the descendants of uranium, such as
238
U-
230
Th, which have
been amazingly successful at dating corals and cave deposits.
4.2.1 The potassium-argon method
This is the workhorse of geochronology. The method relies on the potassium-40 nucleus
capturing a K-shell electron so that
40
K
e
−
→
40
Ar. The radioactive constant or prob-
+
10
−
11
y
−
1
. Potassium-40
ability of decay per unit time for this process is
λ
ε
=
5.81
×
β
−
process into
40
Ca (dual decay), for which the radioactive
also decays by an ordinary
10
−
10
y
−
1
. The proportion of
40
K atoms taking the
40
Ar pathway
is equal to the relative probability
constant is
λ
β
=
4.96
×
λ
ε
/
(
λ
ε
+
λ
β
) or 10.5%. Using
N
for the number of
nuclides, equation
(4.5)
then becomes:
N
40
K
(
t
)
e
(
λ
ε
+
λ
β
)
1
λ
ε
λ
ε
+
λ
β
t
N
40
Ar
(
t
)
=
N
40
Ar
(0)
+
−
(4.25)
with the sum of probabilities of decay, by one or other pathway, in the exponential, and
the proportion of daughter nuclides that are atoms of
40
Ar in the factor term. The princi-
ple behind the method is that the
40
Ar
0
term can be neglected relative to the second term.
Although argon is present in notable quantities in the atmosphere (1%) and in the inter-
stitial gases of rocks, this inert gas is not very soluble at ambient pressure in melts and
minerals since it forms only weak van der Waals'-type bonds with mineral ions. At higher
pressure, however, substantial amounts of argon may remain trapped in submarine glasses
or metamorphic minerals (excess argon).
For analysis, argon is extracted from rocks by heating and melting in ultra-vacuum
lines and analyzed with a mass spectrometer. The ultra-vacuum is necessary to prevent
atmospheric contamination. Such contamination inevitably occurs anyway as minerals
adsorb small quantities of atmospheric gases onto their surfaces or in grain fractures. To
obviate this, we use the fact that atmospheric argon has several isotopes and that the atmo-
spheric
40
Ar/
36
Ar ratio is 296. We simply subtract 296 times the quantity of
36
Ar measured,
only be done with precision when radiogenic
40
Ar is not dominated by that originating in
the atmosphere: measuring young ages for rocks with a low potassium content is therefore
a technical feat of skill. It is thanks to the potassium-argon method that the scale of mag-
netic reversals can be calibrated, as it is the only method for dating the lava on which the
paleomagnetic measurements are made. It will be seen when studying the thermal history
