Argon is a noble gas. The main isotopes of argon in terrestrial systems
are 40Ar (99.6%), 36Ar (0.337%), and 38Ar
(0.063%). Naturally occurring 40K decays to stable 40Ar
(11.2%) by electron capture and by positron emission, and decays to stable
40Ca (88.8%) by negatron emission; 40K has a half-life
of 1.250 x 109 years.
Most of the argon isotope literature deals with measurement of 40Ar
for use in K-Ar age-dating of rocks. The conventional K-Ar dating method
depends on the assumption that the rocks contained no argon at the time
of formation and that all the subsequent radiogenic argon (i.e., 40Ar)
was quantitatively retained. Minerals are dated by measurement of the concentration
of potassium, and the amount of radiogenic 40Ar that has accumulated.
The minerals that are best suited for dating include biotite, muscovite,
and plutonic/high grade metamorphic hornblende, and volcanic feldspar;
whole rock samples from volcanic flows and shallow instrusives can also
be dated if they are unaltered (Faure, 1986). For a discussion of K-Ar
dating see Dalrymple and Lanphere (1969) and Faure (1986).
Under some circumstances the requirements for successful K-Ar dating
may be violated. For example, if 40Ar is lost by diffusion while
the rock cooled, the age-dates represent the time elapsed since the rock
cooled sufficiently for diffusive losses to be insignificant. Or if excess
40Ar is present in the rock, the calculated age-dates are too
old. The 40Ar/39Ar dating method can overcome these
limitations of conventional K-Ar dating, and has the added advantage that
potassium and argon are determined on the same sample and that only measurements
of the isotopic ratios of argon are required. The method is suitable for
use with small and precious samples, such as extraterrestrial materials.
The 40Ar/39Ar dating method is based on the formation
of 39Ar as a result of the intentional irradiation of K-bearing
samples within a nuclear reactor. The bombardment produces various isotopes
of argon from K, Ca, and Cl., but the dominant source of 39Ar
is from 39K. Radioactive 39Ar decays back to 39K
by beta emission with a half-life of 269 years, but the decay is slow compared
to the analysis time and can be ignored (Faure, 1986). The principal advantage
of 40Ar/39Ar dating is that argon can be released
partially by stepwise heating of irradiated samples, producing a spectrum
of dates related to the thermal history of the rock. The principles of
40Ar/39Ar dating are discussed by Dalrymple and Lanphere
(1971, 1974) and Faure (1986).
In the atmosphere, 39Ar is produced by cosmic ray activity,
primarily with 40Ar. In the subsurface environment, it is also
produced through neutron-capture by 39K or alpha emission by
calcium (Zito and Davis, 1981). By virtue of its being a noble gas, it
is relatively unreactive. Argon-39 has been used for a number of applications,
primarily ice coring. It has also been used for ground water dating (Oeschger
et al., 1974). There is hope for expanding the use of this isotope for
hydrologic studies in the future because its half-life falls between that
of 3H and 14C (Florkowski and Rózanski, 1986);
it is therefore useful in bridging the half-life gap between the latter
two isotopes. Thus far, additional input of 39Cl from lithospheric
sources has hampered its use in this field (Fabryka- Martin, 1988).
Argon-37 is produced from the decay of calcium-40, the result of subsurface
nuclear explosions. Its half-life is 35 days. It has not been used for
hydrologic studies; however, it has been used to study atmospheric circulation.
Source of text: This review was assembled by Dan Snyder and Carol
Kendall, primarily drawing upon Faure (1986) and Fabryka-Martin (1988).
||Dalrymple, G. B. and Lanphere, M.A. (1969) Potassium-Argon
dating. W. H. Freeman, San Francisco, 258 p.
||Dalrymple, G. B. and Lanphere, M.A. (1971) 40Ar/39Ar technique
of K-Ar dating: a comparison with the conventional technique,
and Plan. Sci. Letters, v. 12, 300-308.
||Dalrymple, G. B. and Lanphere, M.A. (1974) 40Ar/39Ar age spectra
of some undisturbed terrestrial samples, Geochim. Cosmochim.
Acta, v. 38, 715-738.
||Faure, G. (1986). The K-Ar method of dating, in Principles
of Isotope Geology, second edition, John Wiley, New York,
||Faure, G. (1986). The 40Ar/39Ar method of dating, in
Principles of Isotope Geology, second edition, John Wiley,
New York, p. 93-116.
||Fabryka-Martin, J. T. (1988) Production of Radionuclides
in the Earth and their Hydrogeologic Significance, with Emphasis
on Chlorine-36 and Iodine-129, University of Arizona, PhD.
thesis. 400 pp.
||Florkowski, T. and Rózanski, K. (1986). Radioactive
Noble Gases in the Terrestrial Environment, in Handbook of
Environmental Geochemistry, Vol. 2b (Eds. P. Fritz and J.-Ch.
Fontes), Elsevier Science, New York. pp. 481-506.
||Loosli, H. H. (1989). "Argon-39: a tool to investigate
ocean water circulation and mixing", in Handbook of
Environmental Geochemistry, Vol. 3 (Eds. P. Fritz and J.-Ch.
Fontes), Elsevier Science, New York. pp. 385-392.
||Loosli, H. H. (1992) Applications of 37Ar, 39Ar and 85Kr in
hydrology, oceanography, and atmospheric studies: Current state
of the art, in Isotopes of Noble Gases as Tracers in Environmental
Studies; Proceedings of a Consultants Meeting, International
Atomic Energy Agency, Vienna. pp.73-85
||Oeschger, H., Gugelman, A., Loosi, H., Schotterer, U., Siegenthaler,
U., and Wiest, W. (1974). 39Ar dating of groundwater, in Isotope
Techniques in Groundwater Hydrology, IAEA, Vienna. pp. 179-190.
Zito, R. R., and Davis, S. N. (1981). Subsurface production
of argon and dating of ground water, Appendix 10.12, in Workshop
on Isotope Hydrology Applied to the Evaluation of Deeply Buried
Repositories for Radioactive Wastes, (Ed. S. N. Davis),
Office of Nuclear Waste Isolation, Battelle Memorial Institute.