Chronological Methods 9 - Potassium-Argon Dating
http://id-archserve.ucsb.edu/Anth3/Courseware/Chronology/09_Potassium_Argon_Dating.html

Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40). By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.

How Does the Reaction Work?

Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass). One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40). These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon-40 (Ar-40), an inert gas. For every 100 K-40 atoms that decay, 11 become Ar-40.

How is the Atomic Clock Set?

When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.

The Decay Profile

In this simulation, a unit of molten rock cools and crystallizes. The ratio of K-40 to Ar-40 is plotted. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C-14 graph. Click on the "Show Movie" button below to view this animation.

How are Samples Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

Limitations on K-Ar Dating

The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution. As with any dating technique, there are some significant limitations.

• The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation. For this reason, only trained geologists should collect the samples in the field.
• This technique is most useful to archaeologists and paleoanthropologists when lava flows or volcanic tuffs form strata that overlie strata bearing the evidence of human activity. Dates obtained with this method then indicate that the archaeological materials cannot be younger than the tuff or lava stratum. Because the materials dated using this method are NOT the direct result of human activity, unlike radiocarbon dates for example, it is critical that the association between the igneous/volcanic beds being dated and the strata containing human evidence is very carefully established.
• As the simulation of the processing of potassium-argon samples showed, the standard deviations for K-Ar dates are so large that resolution higher than about a million years is almost impossible to achieve. By comparison, radiocarbon dates seem almost as precise as a cesium clock! Potassium-argon dating is accurate from 4.3 billion years (the age of the Earth) to about 100,000 years before the present. At 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40, pushing the limits of present detection devices. Eventually, potassium-argon dating may be able to provide dates as recent as 20,000 years before present.