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.
![[Show Movie]](../../Images/Btns/ShowMovie.gif){kind=small}
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.
Links
Helpful References
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