Chemistry Reference
In-Depth Information
method, rocks, are directly related not to human activities but to geologic
events. It is important, therefore, when using the potassium-argon method,
that the association between the rocks dated and the human evidence to
which the rock is related be carefully established. Bearing in mind this
limitation, potassium-argon is the only viable method for dating very old,
archaeologically related rocks such as hornblende, certain kinds of feldspar,
lava, some natural glass, and types of clay (Walther 1997). A useful applica-
tion of the technique in archaeological studies is, for example, to date vol-
canic tuff or lava flows overlying layers bearing evidence of early hominids
and/or their activity; such dates provide evidence that the layers bearing
archaeological evidence are older than the overlaying lava or tuff.
TEXTBOX
15
POTASSIUM-ARGON AND ARGON-ARGON DATING
Potassium-Argon Dating
Potassium (K) is one of the most abundant elements (2.4% by mass) in
the earth's crust, where it occurs in three isotopic forms:
potassium-39
,
potassium-40
, and
potassium-41
.
Potassium-40
, which is radioactive, is the
least abundant of the three isotopes, making up a very small fraction, just
over 0.01%, of the total amount of potassium; one out of every 10,000
potassium atoms is radioactive potassium-40. The isotope decays by two
different pathways: (1) 88% of the potassium-40 decays to
calcium-40
, and
(2) the remaining 12% decays to
argon-40
, a stable isotope of argon also
known as
radiogenic argon
(see Fig. 11). The decay of potassium-40 to
argon-40 is the relevant decay pathway in the potassium-argon (K-Ar)
dating method.
Potassium-40 decays, and as a consequence, in any mineral or rock con-
taining potassium, the concentration of potassium-40 is gradually reduced,
while, provided the rock is impervious to argon, the amount of argon-40
increases. Since the half-life of potassium-40 has been precisely determined
(1.28
10
9
years), measuring the concentration of potassium-40 and argon-
40 in minerals, rock, or volcanic ash enables one to determine their age
(Faure 1986). Thus, if argon remains trapped within the minerals that make
up rock or volcanic ash,
potassium-argon dating
, as the method is known,
provides an invaluable tool for studying early evidence on human evolu-
tion. It should be noted, however, that potassium-argon ages are not
directly related to human activity. They provide maximum time limits, indi-
cating whether dated minerals, rocks or ashes are younger, or older, than
the remains of the activity of humans. If human remains lie between two
layers of volcanic deposits, for example, the potassium-argon dates of the
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