Periodic Table--Argon

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).