average is then calculated from these measurements and
used to calculate the sample value.
Age Calculation of a Sample
Here are the measurements obtained for 1 sanidine crystal
irradiated for 90 min in the Osiris reactor (CEA Saclay):
40
Ar
39 Ar
38 Ar
37 Ar
36 Ar
Measured (mV)
2.491 Â 10
−03
7.969 Â 10
−04
1.241 Â 10
−05
4.612 Â 10
−07
3.136 Â 10
−06
Blank (mV)
1.791 Â 10
−04
1.515 Â 10
−07
7.158 Â 10
−08
1.451 Â 10
−07
1.554 Â 10
−07
Corrected
measurement
2.312 Â 10
−03
7.967 Â 10
−04
1.234 Â 10
−05
3.161 Â 10
−07
2.981 Â 10
−06
By combining the expression of J (Eq. 5.9) with
Eq. 5.10, the age calculation gives:
t e ¼
1
k
ln 1 þ J
40
Ar
Ã
39 Ar K
!
with J = 6.530 Â 10
−4 (calculated for this sample);
k = 5.543 Â 10
−10 (constant for total decay of
40 K);
R e = 1.790 8 (see calculation of R e above with
[
40 Ar/
36 Ar] A = 296.1);
t e = 1/(5.543 Â 10
−10 ) Â ln(1 + 6.530 Â 10
−4
 1.790
8) = 2.108 Ma.
Advantages and Limitations of the
40
K/
40
Ar
and
40
Ar/
39
Ar Methods
The table below summarizes the advantages and limitations
of both methods.
Method
40
K/
40
Ar
40
Ar/
39
Ar
Advantages
• Rapid implementation
• No need for prior
irradiation of samples
• Precise measurement
of low amounts of
40
Ar
* (well-suited to
young basalts of
mid-oceanic ridges)
• Basic assumptions can
be verified (age
spectrum, isochrons)
• Dating possible on
very small sample
sizes (grain by grain
dating well suited to
tephra)
Limitations
• The basic assumptions
(initial
40
Ar/
36
Ar = 298.56,
evolution in a closed
system) for application
of the clock are not
verified
• Large weight (>1 g) of
sample required
These two points
prohibit the dating of
tephra by the K-Ar
method
• Pre-irradiation leads to
corrections
(interference of
masses)
• The recoil effect makes
dating of very
fine-grained samples
or ones with a glassy
texture complicated
Application: Example of the Dating
of the Laschamp Event
In the chapter on magnetic stratigraphy (Chap. 7), the
importance of the dating of geomagnetic events is discussed.
Here, we will show how the Laschamp excursion could be
correctly dated with a high degree of precision.
The dating of this excursion was obtained by a
geochronological study combining the
40 K/
40 Ar and
40 Ar/
39 Ar methods applied to two lavas from the Massif
Central (Guillou et al. 2004). Prior to this study, estimates of
ages were imprecise and inconsistent with the ages deduced
from other means of dating, such as astronomical calibration.
Two lava flows have been subjected to paleomagnetic
and geochronological study. One of these comes from the
‘Puy de Laschamp’ part of the Chaine des Puys, located in
the French Massif Central. The second, called ‘the Olby
flow’ comes from the ‘Puy de Barme’ also part of the Chaine
des Puys.
For each sample, the K-Ar ages (unspiked method) are
calculated from two independent measures of potassium and
three, also independent, measures of argon. The ages
obtained for the Laschamps lava flow (41.5 ± 1.9 ka) and
that of Olby (41.4 ± 1.9 ka) are identical at the two sigma
level. The weighted average of these two values gives an age
of 41.4 ± 1.4 ka. As for
40 Ar/
39 Ar ages (Fig. 5.12), seven
experiments out of thirteen give consistent age spectra, for
which 100% of the extracted gas could be used to define a
plateau age. For the other six experiments, between 76 and
96% of the extracted gas was used to define a plateau age.
Furthermore, the intercept values calculated from the inverse
isochron diagrams are equivalent to the atmospheric ratio.
This indicates that the age determinations are not marred by
error due to either a loss or gain of argon. The weighted
average for isochron ages for the two sampling sites for the
Laschamps lava flow were 39.4 ± 2.6 ka and
38.3 ± 2.6 ka. The Olby lava flow has an age of
39.2 ± 4.9 ka. The combination of these three ages give a
weighted average of 38.9 ± 1.7 ka.
The K-Ar and
40 Ar/
39 Ar measurements are compatible at
the two sigma level. As these two flows recorded the same
paleomagnetic excursion, this is dated to 40.4 ± 1.1 ka.
Note that if we take the uncertainty (2.4%) on the potassium
decay constant into account, the error in age goes from 1.1 to
2.0 ka. Thus, the age retained for the excursion is
40.4 ± 2.0 ka. This age is comparable to that obtained by
independent chronological methods (see Chap. 7). A new
study (Laj et al. 2014), combining K-Ar and
40 Ar/
39 Ar
dating, associated with paleomagnetism, was applied to a
larger number of volcanoes from the Chaine des Puys, and
has since made it possible to narrow down the age of the
Laschamps excursion to 41.2 ± 1.6 ka.
5 The
40
K/
40
Ar and
40 Ar/
39
Ar Methods
85
used to calculate the sample value.
Age Calculation of a Sample
Here are the measurements obtained for 1 sanidine crystal
irradiated for 90 min in the Osiris reactor (CEA Saclay):
40
Ar
39 Ar
38 Ar
37 Ar
36 Ar
Measured (mV)
2.491 Â 10
−03
7.969 Â 10
−04
1.241 Â 10
−05
4.612 Â 10
−07
3.136 Â 10
−06
Blank (mV)
1.791 Â 10
−04
1.515 Â 10
−07
7.158 Â 10
−08
1.451 Â 10
−07
1.554 Â 10
−07
Corrected
measurement
2.312 Â 10
−03
7.967 Â 10
−04
1.234 Â 10
−05
3.161 Â 10
−07
2.981 Â 10
−06
By combining the expression of J (Eq. 5.9) with
Eq. 5.10, the age calculation gives:
t e ¼
1
k
ln 1 þ J
40
Ar
Ã
39 Ar K
!
with J = 6.530 Â 10
−4 (calculated for this sample);
k = 5.543 Â 10
−10 (constant for total decay of
40 K);
R e = 1.790 8 (see calculation of R e above with
[
40 Ar/
36 Ar] A = 296.1);
t e = 1/(5.543 Â 10
−10 ) Â ln(1 + 6.530 Â 10
−4
 1.790
8) = 2.108 Ma.
Advantages and Limitations of the
40
K/
40
Ar
and
40
Ar/
39
Ar Methods
The table below summarizes the advantages and limitations
of both methods.
Method
40
K/
40
Ar
40
Ar/
39
Ar
Advantages
• Rapid implementation
• No need for prior
irradiation of samples
• Precise measurement
of low amounts of
40
Ar
* (well-suited to
young basalts of
mid-oceanic ridges)
• Basic assumptions can
be verified (age
spectrum, isochrons)
• Dating possible on
very small sample
sizes (grain by grain
dating well suited to
tephra)
Limitations
• The basic assumptions
(initial
40
Ar/
36
Ar = 298.56,
evolution in a closed
system) for application
of the clock are not
verified
• Large weight (>1 g) of
sample required
These two points
prohibit the dating of
tephra by the K-Ar
method
• Pre-irradiation leads to
corrections
(interference of
masses)
• The recoil effect makes
dating of very
fine-grained samples
or ones with a glassy
texture complicated
Application: Example of the Dating
of the Laschamp Event
In the chapter on magnetic stratigraphy (Chap. 7), the
importance of the dating of geomagnetic events is discussed.
Here, we will show how the Laschamp excursion could be
correctly dated with a high degree of precision.
The dating of this excursion was obtained by a
geochronological study combining the
40 K/
40 Ar and
40 Ar/
39 Ar methods applied to two lavas from the Massif
Central (Guillou et al. 2004). Prior to this study, estimates of
ages were imprecise and inconsistent with the ages deduced
from other means of dating, such as astronomical calibration.
Two lava flows have been subjected to paleomagnetic
and geochronological study. One of these comes from the
‘Puy de Laschamp’ part of the Chaine des Puys, located in
the French Massif Central. The second, called ‘the Olby
flow’ comes from the ‘Puy de Barme’ also part of the Chaine
des Puys.
For each sample, the K-Ar ages (unspiked method) are
calculated from two independent measures of potassium and
three, also independent, measures of argon. The ages
obtained for the Laschamps lava flow (41.5 ± 1.9 ka) and
that of Olby (41.4 ± 1.9 ka) are identical at the two sigma
level. The weighted average of these two values gives an age
of 41.4 ± 1.4 ka. As for
40 Ar/
39 Ar ages (Fig. 5.12), seven
experiments out of thirteen give consistent age spectra, for
which 100% of the extracted gas could be used to define a
plateau age. For the other six experiments, between 76 and
96% of the extracted gas was used to define a plateau age.
Furthermore, the intercept values calculated from the inverse
isochron diagrams are equivalent to the atmospheric ratio.
This indicates that the age determinations are not marred by
error due to either a loss or gain of argon. The weighted
average for isochron ages for the two sampling sites for the
Laschamps lava flow were 39.4 ± 2.6 ka and
38.3 ± 2.6 ka. The Olby lava flow has an age of
39.2 ± 4.9 ka. The combination of these three ages give a
weighted average of 38.9 ± 1.7 ka.
The K-Ar and
40 Ar/
39 Ar measurements are compatible at
the two sigma level. As these two flows recorded the same
paleomagnetic excursion, this is dated to 40.4 ± 1.1 ka.
Note that if we take the uncertainty (2.4%) on the potassium
decay constant into account, the error in age goes from 1.1 to
2.0 ka. Thus, the age retained for the excursion is
40.4 ± 2.0 ka. This age is comparable to that obtained by
independent chronological methods (see Chap. 7). A new
study (Laj et al. 2014), combining K-Ar and
40 Ar/
39 Ar
dating, associated with paleomagnetism, was applied to a
larger number of volcanoes from the Chaine des Puys, and
has since made it possible to narrow down the age of the
Laschamps excursion to 41.2 ± 1.6 ka.
5 The
40
K/
40
Ar and
40 Ar/
39
Ar Methods
85