Potassium has three naturally occurring isotopes: 39K (93.3%),
40K (0.01%) and 41K (6.7%). 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. The decay
of 40K to 40Ar is commonly used as a method for dating
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, i.e., closed system. 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), Faure (1986), and Dicken
Outside of dating, K isotopes have been used extensively in studies
of weathering (Moldan and Cerny, 1993); K isotopes have also be used for
nutrient cycling studies because K is a macro-nutrient required for life.
Source of text: This review was assembled by Dan Snyder, primarily
from Faure (1986).
||Dalrymple, G.B. and Lanphere, M.A. (1969). Potassium-Argon dating. W.H. Freeman, San Francisco, 258 p.
||Dicken, A.P. (1995). Radiogenic Isotope Geology. Cambridge University
Press, New York, 452 p.
||Faure, G. (1986). "The K-Ar method of dating", in Principles
of Isotope Geology, second edition, John Wiley and Sons, New York,
||Hunziker, J. C. (1979). "Potassium argon dating", In: E. Jäger
and J.C. Hunziker (Eds.), Lecutres in Isotope Geology, Springer-
Verlag, Berlin. pp. 52-76.
||Moldan, B. and Cerny, J. (Eds.)(1993). Biogeochemistry of Small Catchments--A
Tool for Environmental Research, Scientific Committee on Problems of
the Environment, John Wiley and Sons, New York. 419 p.