Potassium–Argon (K–Ar) dating

Basic principles

K–Ar dating measures the accumulation of radiogenic 40Ar produced by decay of 40K inside a mineral or rock. When molten rock cools, any argon gas present usually escapes; after solidification the mineral lattice traps newly produced 40Ar. By measuring the ratio of 40Ar to the remaining 40K, and knowing the decay rate, an age can be calculated. Practical laboratory procedures for measuring concentrations and correcting for atmospheric argon are described in many analytical guides (measurement methods).

Materials and sample selection

Suitable material includes rapidly cooled volcanic rocks, volcanic glass, tephra and potassium-bearing minerals. Typical minerals used are micas, orthoclase and some clays and evaporites. Ideal samples are those that were heated above a closure temperature so that previous argon was removed and that remained closed systems afterwards. Common sample types and preparation steps are:

• Minerals: biotite, muscovite, sanidine, orthoclase (micas, feldspars).
• Volcanic materials: lavas and tephra that cooled quickly and trapped argon.
• Preparation: crushing, mineral separation, irradiation or mass-spectrometric analysis to determine potassium and argon contents (argon analysis).

History and technical developments

Developed in the mid-20th century, K–Ar dating helped establish absolute ages for volcanic sequences and played a key role in calibrating the geomagnetic reversal time scale. Early worker contributions and methodological refinements are discussed in the literature (method history). Later improvements led to the argon–argon (40Ar/39Ar) method, which allows single-sample irradiation and incremental heating to check for argon loss or excess argon (40Ar/39Ar, incremental heating).

Applications and importance

K–Ar dating is widely used in volcanology, tectonics, paleomagnetism and archaeology when ages are older than the range of radiocarbon. Volcanic rocks dated by K–Ar can anchor biologic, climatic and geomagnetic events in time. For example, the technique contributed to constructing the geomagnetic polarity time scale (geomagnetic reversals) and is used to date tephra layers that serve as regional time markers (tephrochronology). It also provides constraints on the timing of metamorphism and cooling in orogenic belts (metamorphic studies).

Limitations and common issues

Reliable ages require that the sample remained a closed system after cooling. Argon loss due to later heating or chemical alteration produces ages that are too young, while inherited "excess argon" trapped at formation can give ages that are too old. The method is less useful for very young materials because the slow decay of 40K means little 40Ar accumulates on short time scales (the effective useful range is suitable for geological, not recent archaeological, times). Analytical corrections and cross-checks with other methods are advisable; see discussions of uncertainties and best practices (analytical corrections, closure temperature, argon loss).