5
The
40 K/
40 Ar and
40 Ar/
39 Ar Methods
Hervé Guillou, Sébastien Nomade, and Vincent Scao
The
40 K/
40 Ar method and its variant,
40 Ar/
39 Ar, are based on
the natural radioactive decay of
40 K, one of the isotopes of
potassium, in
40 Ar, one of the isotopes of argon.
40
K
decreases in
40 Ar
* (the * symbol indicates that this is a
radiogenic isotope) with a period of 1.25 Â 10
9 years,
according to the law of radioactive decay N = N 0 e
−kt
. In
other words, if we consider a closed system, containing at an
initial time (t 0 ) N 0 atoms of
40
K, then N 0 /2 atoms of
40 K will
remain in the system after 1.25 Â 10
9 years. This gives us
an indication of the geochronological application. If, in a
geological sample, both the number of parent atoms
remaining (
40 K) and the number of daughter atoms formed
(
40 Ar
* ) can be measured, then it is possible to calculate the
age of formation of this sample. The relatively high abundance of the isotope
40 K (K is the seventh most abundant
element on Earth), combined with a low decay rate, makes
the
40 K/
40 Ar method and its variant
40 Ar/
39 Ar two of the
most widely used geochronological tools in Earth Sciences.
They are applicable to various geological materials and
cover a wide range of ages, given the long period of
40 K.
Already in 1921, Aston, using a mass spectrograph,
proved the existence of two isotopes of potassium (
39
K and
41 K). In 1935, Klemperer, and also Neuman and Walker,
experimentally demonstrated the natural radioactive decay of
40 K to
40 Ca and
40 Ar
* . In 1948, Aldrich and Nier confirmed
the radiogenic origin of argon
40 Ar
* . They experimentally
determined the
40 Ar/
36 Ar ratio of several potassic minerals
and compared it to that of the atmosphere, assuming it to be
constant and equal to 298.56 (this updated value, determined
by Lee et al. (2006), replaces the previous one of 295.5
established by Steiger and Jäger (1977)). As the ratios
obtained were superior to that of the atmosphere, the source
of argon
40 Ar
* by radioactive decay of
40 K was demonstrated. In parallel, understanding of the decay constant of
40 K became more accurate. Aldrich and Nier could therefore
see the potential of the
40 K/
40 Ar pair for the dating of rocks.
The K-Ar clock is based on the principles of radioactive
decay and the accumulation of a daughter isotope. However,
studies subsequent to Aldrich and Nier’s work showed that
there were many causes of disturbance in the K/Ar clock.
Among these are the inability of certain rocks or minerals to
retain all of the radiogenic argon-40 (
40 Ar
* ), or the presence
of ‘excess argon’ in some samples. Consequently, for a K-Ar
age to be accepted as correct, the following must be true:
1. when starting the clock (at time zero t 0 ), the
40 Ar/
36 Ar
ratio in the sample is the same as that of the atmosphere
(298.56), in other words that
40 Ar
* = 0;
2. and between t 0 and the moment the sample is dated, it
behaves as a closed system with regard to
40 K and
40 Ar.
The conventional K/Ar method does not allow verification of these two major assumptions. To remedy this, the
40 Ar/
39 Ar variant was developed by Wänke and König
(1959) and Merrihue (1965), which showed that the
40 K/
40 Ar ages can be obtained by irradiating samples of
rocks or minerals. When a sample is subjected to a neutron
flux in a reactor, some
39 K becomes
39 Ar. The measurement
of the
39 Ar content by a counting method calculates the
number of parent atoms (
40 K) remaining in the sample, since
we know the relative abundances of different isotopes of
potassium (see below). Using the same counting method, the
number of
40 Ar isotopes present in the sample can also be
measured. However, this approach is unsatisfactory because
it does not allow for necessary corrections related to the
process of irradiation to be made, as we shall see later, and
the precision achieved on the different concentrations of
argon isotopes is insufficient. The crucial work for the
40 Ar/
39 Ar method was carried out by Merrihue in 1965. He
showed that the argon
39 Ar generated in a nuclear reactor
from the
39 K of a sample can be measured precisely by mass
spectrometry. This
39 Ar, derived from the
39
K, is annotated
as
39 Ar K . In addition, the other isotopes of argon,
40 Ar and
H. Guillou (&) Á S. Nomade Á V. Scao
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
91190 Gif-sur-Yvette, France
e-mail: herve.guillou@lsce.ipsl.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_5
73
The
40 K/
40 Ar and
40 Ar/
39 Ar Methods
Hervé Guillou, Sébastien Nomade, and Vincent Scao
The
40 K/
40 Ar method and its variant,
40 Ar/
39 Ar, are based on
the natural radioactive decay of
40 K, one of the isotopes of
potassium, in
40 Ar, one of the isotopes of argon.
40
K
decreases in
40 Ar
* (the * symbol indicates that this is a
radiogenic isotope) with a period of 1.25 Â 10
9 years,
according to the law of radioactive decay N = N 0 e
−kt
. In
other words, if we consider a closed system, containing at an
initial time (t 0 ) N 0 atoms of
40
K, then N 0 /2 atoms of
40 K will
remain in the system after 1.25 Â 10
9 years. This gives us
an indication of the geochronological application. If, in a
geological sample, both the number of parent atoms
remaining (
40 K) and the number of daughter atoms formed
(
40 Ar
* ) can be measured, then it is possible to calculate the
age of formation of this sample. The relatively high abundance of the isotope
40 K (K is the seventh most abundant
element on Earth), combined with a low decay rate, makes
the
40 K/
40 Ar method and its variant
40 Ar/
39 Ar two of the
most widely used geochronological tools in Earth Sciences.
They are applicable to various geological materials and
cover a wide range of ages, given the long period of
40 K.
Already in 1921, Aston, using a mass spectrograph,
proved the existence of two isotopes of potassium (
39
K and
41 K). In 1935, Klemperer, and also Neuman and Walker,
experimentally demonstrated the natural radioactive decay of
40 K to
40 Ca and
40 Ar
* . In 1948, Aldrich and Nier confirmed
the radiogenic origin of argon
40 Ar
* . They experimentally
determined the
40 Ar/
36 Ar ratio of several potassic minerals
and compared it to that of the atmosphere, assuming it to be
constant and equal to 298.56 (this updated value, determined
by Lee et al. (2006), replaces the previous one of 295.5
established by Steiger and Jäger (1977)). As the ratios
obtained were superior to that of the atmosphere, the source
of argon
40 Ar
* by radioactive decay of
40 K was demonstrated. In parallel, understanding of the decay constant of
40 K became more accurate. Aldrich and Nier could therefore
see the potential of the
40 K/
40 Ar pair for the dating of rocks.
The K-Ar clock is based on the principles of radioactive
decay and the accumulation of a daughter isotope. However,
studies subsequent to Aldrich and Nier’s work showed that
there were many causes of disturbance in the K/Ar clock.
Among these are the inability of certain rocks or minerals to
retain all of the radiogenic argon-40 (
40 Ar
* ), or the presence
of ‘excess argon’ in some samples. Consequently, for a K-Ar
age to be accepted as correct, the following must be true:
1. when starting the clock (at time zero t 0 ), the
40 Ar/
36 Ar
ratio in the sample is the same as that of the atmosphere
(298.56), in other words that
40 Ar
* = 0;
2. and between t 0 and the moment the sample is dated, it
behaves as a closed system with regard to
40 K and
40 Ar.
The conventional K/Ar method does not allow verification of these two major assumptions. To remedy this, the
40 Ar/
39 Ar variant was developed by Wänke and König
(1959) and Merrihue (1965), which showed that the
40 K/
40 Ar ages can be obtained by irradiating samples of
rocks or minerals. When a sample is subjected to a neutron
flux in a reactor, some
39 K becomes
39 Ar. The measurement
of the
39 Ar content by a counting method calculates the
number of parent atoms (
40 K) remaining in the sample, since
we know the relative abundances of different isotopes of
potassium (see below). Using the same counting method, the
number of
40 Ar isotopes present in the sample can also be
measured. However, this approach is unsatisfactory because
it does not allow for necessary corrections related to the
process of irradiation to be made, as we shall see later, and
the precision achieved on the different concentrations of
argon isotopes is insufficient. The crucial work for the
40 Ar/
39 Ar method was carried out by Merrihue in 1965. He
showed that the argon
39 Ar generated in a nuclear reactor
from the
39 K of a sample can be measured precisely by mass
spectrometry. This
39 Ar, derived from the
39
K, is annotated
as
39 Ar K . In addition, the other isotopes of argon,
40 Ar and
H. Guillou (&) Á S. Nomade Á V. Scao
Laboratoire des Sciences du Climat et de l’Environnement,
LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay,
91190 Gif-sur-Yvette, France
e-mail: herve.guillou@lsce.ipsl.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_5
73