Link to USGS home page

Resources on Isotopes

Periodic Table--Potassium

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 (1995).

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, pp. 66-92.
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.
Related Links
Periodic Table
Fundamentals of Stable Isotope Geochemistry
General References
Isotope Publications
Please contact Carol Kendall ( for questions and comments regarding this page.
This page was last changed in January 2004.
To the USGS Home Page
To the Water Resources Information Home Page