Radiometric dating method
Argon?argon
(or
40
Ar/
39
Ar
)
dating
is a
radiometric dating
method invented to supersede
potassium?argon (K/Ar) dating
in accuracy. The older method required splitting samples into two for separate
potassium
and
argon
measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of
argon isotopes
.
40
Ar/
39
Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (
39
K) into the radioactive
39
Ar. As long as a standard of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon isotopes to calculate the
40
K/
40
Ar* ratio, and thus to calculate the age of the unknown sample.
40
Ar* refers to the
radiogenic
40
Ar, i.e. the
40
Ar produced from radioactive decay of
40
K.
40
Ar* does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the
36
Ar (which is assumed to be of atmospheric origin) and assuming that
40
Ar is found in a constant ratio to
36
Ar in atmospheric gases.
Method
[
edit
]
The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce
39
Ar from
39
K via the
(n-p) reaction
39
K(n,p)
39
Ar. The sample is then degassed in a high-vacuum
mass spectrometer
via a laser or resistance furnace. Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and radiogenic gases like argon and helium, generated from regular radioactive decay over geologic time. The abundance of
40
Ar* increases with the age of the sample, though the rate of increase decays exponentially with the half-life of
40
K, which is 1.248 billion years.
Age equation
[
edit
]
The age of a sample is given by the age equation:
where λ is the radioactive
decay constant
of
40
K (approximately 5.5 x 10
?10
year
?1
, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the
40
Ar*/
39
Ar ratio. The J factor relates to the
fluence
of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of
39
K to
39
Ar than a less dense one.
Relative dating only
[
edit
]
The
40
Ar/
39
Ar method only measures relative dates. In order for an age to be calculated by the
40
Ar/
39
Ar technique, the J parameter must be determined by irradiating the unknown sample along with a sample of known age for a standard. Because this (primary) standard ultimately cannot be determined by
40
Ar/
39
Ar, it must be first determined by another dating method. The method most commonly used to date the primary standard is the
conventional K/Ar technique
.
[1]
An alternative method of calibrating the used standard is astronomical tuning (also known as
orbital tuning
), which arrives at a slightly different age.
[2]
Applications
[
edit
]
The primary use for
40
Ar/
39
Ar geochronology is dating metamorphic and igneous minerals.
40
Ar/
39
Ar is unlikely to provide the age of intrusions of
granite
as the age typically reflects the time when a mineral cooled through its
closure temperature
. However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral. Dating of movement on
fault
systems is also possible with the
40
Ar/
39
Ar method. Different minerals have different closure temperatures;
biotite
is ~300°C,
muscovite
is about 400°C and
hornblende
has a closure temperature of ~550°C. Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures. Thus, although a crystallization age is not recorded, the information is still useful in constructing the thermal history of the rock.
Dating minerals
may
provide age information on a rock, but assumptions must be made. Minerals usually only record the
last time
they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion. Thus, discretion and interpretation of age dating is essential.
40
Ar/
39
Ar geochronology assumes that a rock retains all of its
40
Ar after cooling past the
closing temperature
and that this was properly sampled during analysis.
This technique allows the errors involved in K-Ar dating to be checked. Argon?argon dating has the advantage of not requiring determinations of potassium. Modern methods of analysis allow individual regions of crystals to be investigated. This method is important as it allows crystals forming and cooling during different events to be identified.
Recalibration
[
edit
]
One problem with argon-argon dating has been a slight discrepancy with other methods of dating.
[3]
Work by Kuiper et al. reports that a correction of 0.65% is needed.
[4]
Thus the
Cretaceous?Paleogene extinction
(when the dinosaurs died out)?previously dated at 65.0 or 65.5 million years ago?is more accurately dated to 66.0-66.1 Ma.
See also
[
edit
]
References
[
edit
]
- ^
"New Mexico Geochronology Research Laboratory: K/Ar and
40
Ar/
39
Ar Methods"
. New Mexico Bureau of Geology and Mineral Resources. Archived from
the original
on 2017-08-03
. Retrieved
2008-09-16
.
- ^
Kuiper, K. F.; Hilgen, F. J.; Steenbrink, J.; Wijbrans, J. R. (2004).
"
40
Ar/
39
Ar ages of tephras intercalated in astronomically tuned Neogene sedimentary sequences in the eastern Mediterranean"
(PDF)
.
Earth and Planetary Science Letters
.
222
(2): 583?597.
Bibcode
:
2004E&PSL.222..583K
.
doi
:
10.1016/j.epsl.2004.03.005
.
- ^
Renne, P. R. (1998). "Absolute Ages Aren't Exactly".
Science
.
282
(5395): 1840?1841.
doi
:
10.1126/science.282.5395.1840
.
S2CID
129857264
.
- ^
Kuiper, K. F.; Deino, A.; Hilgen, F. J.; Krijgsman, W.; Renne, P. R.; Wijbrans, J. R. (2008). "Synchronizing Rock Clocks of Earth History".
Science
.
320
(5875): 500?504.
Bibcode
:
2008Sci...320..500K
.
doi
:
10.1126/science.1154339
.
PMID
18436783
.
S2CID
11959349
.
External links
[
edit
]