Q: Independent confirmation of K-Ar dating?
Mark Patterson
Hi all,
I'm doing a study of potassium-argon dating, used for dating fossils. A
lot of
evolution is based on K-Ar dates, so I woould like to hear of are any
independent measures to confirm or calibrate or invalidate that method.
For example, what dates do we get from known eruptions, such as the one
that destroyed Pompeii or Saint Pierre? Or the eruptions of Krakatoa,
Mount St
Helens or Ruapehu, or any others in recent history? There must be dozens
of examples that have precise dates and could be used to evalaute K-Ar
as a dating method.
Please e-mail me any replies,
Mark Patterson
Andrew MacRae
In article <351207...@enternet.com.au> Mark Patterson
Hi Mark:
First of all, I recommend Chris Stassen's "Isochron Dating FAQ",
which is at:
http://www.talkorigins.org/faqs/isochron-dating.html
And take a look at the references cited in there if you have not
already, particularly Dalrymple's "The Age of the Earth", and Gunter
Faure's "Principles of Isotope Geology". They discuss the principles and
history of the method.
This is a good question, however, you have to keep one crucial
fact in mind: K decays very slowly, and for eruptions in historical
times, not enough time has expired yet for much radioactive decay to have
occurred. This makes it very difficult to pick out the "age signal"
through the "noise" caused by tiny amounts of initial Ar present in
typical volcanic rock samples, and even to pick out the Ar due to
radioactive decay versus the Ar in the ambient atmosphere. To do it at
all takes extremely careful instrumentation, because you are trying to do
a measurement at the limits of the method. Because of its slow decay
constant, K/Ar is more suited to dating much older rocks. Anything
younger than a million years or so (depending upon K content) is pushing
it, unless special techniques are used (see below). The analogy I used
recently was to an effort to time a 100 metre dash with a clock that has
only hour hands, or a calendar, instead of using a more appropriate
chronometer like a stopwatch (which, correspondingly, would not be good
for measuring times over periods of months or years). To date something
comparably young, it is better to use isotopic systems with more rapid
decay, like C14 and several others.
Despite these problems, instrumentation and calibration of K/Ar
methods has improved tremendously in the last few decades. Conventional
K/Ar, where the Ar isotopes are measured in a mass spectrometer and the K
is measured chemically, have been used for many decades, and are still the
cheapest and simplest method, but have several limitations, including
large sample size and measurements by two different techniques. More
recent methods, like 40Ar/39Ar, measure isotopic ratios on the same
instrument at the same time (much more precise), the Ar can also be heated
out of the sample in a stepwise fashion (so the less resistant Ar-bearing
sites in crystals can be excluded), and improvements in the
instrumentation now mean that tiny samples -- single mineral grains or
even parts of mineral grains -- can be dated to extreme precision.
Isochron techniques (a whole extra concept) also help greatly. This has
now made it possible to extend K/Ar methods (or, more precisely, its
derivatives, like 40Ar/39Ar) into the historical realm. This was not
really possible 20 years ago. It is still pushing the limits of
practicality, and is cutting-edge stuff, but there has been alot of
success recently, and the results basically verify the method, and say a
great deal about Ar isotopic behaviour in volcanic systems (which is
interesting for *much* more than just the age of eruption of the rocks --
it tells you about the behaviour of different mineral phases within the
magma chamber before eruption).
The best example of what you are looking for is probably:
Renne, P.R.; Sharp, W.D.; Deino, A.L.; Orsi, G.; and Civetta, L., 1997
(August 29). 40Ar/39Ar dating into the historical realm: calibration
against Pliny the Younger. Science, v.277, p.1279-1280.
This one dates the 79A.D. eruption of Vesuvius that buried
Pompeii, one of the examples you listed. The eruption occurred about 1918
years ago, and the 40Ar/39Ar isochron age comes out to 1925+-94 years ago.
Here is another example, although this one is not in the historical range,
but it does push the limits of the method, and it talks quite a bit about
the challenges of working with K/Ar dates that young, and inter-method
comparisons:
Yanshao Chen; Smith, P.E.; Evensen, N.M.; York, D.; and Lajoie, K.R., 1996
(Nov. 16). The edge of time: dating young volcanic ash layers with the
40Ar-39Ar laser probe. Science, v.274, p.1176-1178.
And, because you are interested in independent confirmation of the
method, and there is much more to that than just historical eruptions, I
recommend:
Baadsgaard, H.; Lerbekmo, J.F.; and McDougall, I., 1988. A radiometric
age for the Cretaceous-Tertiary boundary based upon K-Ar, Rb-Sr, and U-Pb
ages of bentonites from Alberta, Saskatchewan, and Montana. Canadian
Journal of Earth Sciences, v.25, p.1088-1097.
This one uses multiple methods at multiple localities to date the
boundary between the Cretaceous and Tertiary Periods.
Oh, and I would be remiss if I did not mention:
http://www.icr.org/research/sa/sa-r01.htm
Which is a paper by Steve Austin of the ICR, about his attempts to
conventional K/Ar date the very recently-formed lava dome at Mt. St.
Helens. For comments about the significance of his results, take a look
at the recent thread "Re: a visit to the icr, part 1" in this newsgroup.
If you can not find it on your news server, you can find it by searching
at http://www.dejanews.com/
I hope this answers your question. Good luck with your research.
-Andrew
mac...@agc.bio._NOSPAM_.ns.ca
Chris Stassen
[Posted and E-mailed]
mark...@enternet.com.au wrote:
> I'm doing a study of potassium-argon dating, used for dating fossils.
In general, isotopic dating methods don't apply well to sedimentary
formations (which is where most fossils are found).
> A lot of evolution is based on K-Ar dates,
That is not really true. The succession of fauna, ordering of the
geologic column, and even evolution, were all quite well-established
(mid to late 1800s) long before the technology existed to perform
accurate isotope dating (mid 1900s)... even before scientists were
aware of radioactivity (circa 1890-1900) or the existence of
isotopes (early 1900s).
> so I woould like to hear of are any independent measures to confirm or
> calibrate or invalidate that method. For example, what dates do we get
> from known eruptions, such as the one that destroyed Pompeii or
> Saint Pierre? Or the eruptions of Krakatoa, Mount St Helens or Ruapehu,
> or any others in recent history?
See, for example: Dalrymple, G. Brent, 1969. "40Ar/36Ar Analyses
of Historic Lava Flows" in _Earth and Planetary Science Letters_
vol. 6, pp. 47-55. It lists argon assessments for 26 samples from
several sources of known age. I have a reference to another study
but I have not yet obtained the actual paper, so I can't recommend
it.
> There must be dozens of examples that have precise dates and could
> be used to evalaute K-Ar as a dating method.
Uncertainty in a measured age is a function of uncertainty in the
isotope assessments *and* the half-life of the relevant isotope.
Since K-40 has a fairly long half-life (about a billion years),
it's fairly rare to have age uncertainties less than a few tens
of thousands of years.
Measurements on recent samples indicate that the initial conditions
are usually suitable for accurate long-term K-Ar ages, but in most
cases the uncertainty in the assessment of young ages is greater
than the known ages of the samples (because they are all less than
a few thousand years old).
K-Ar dating shows excellent correlations with other long-term
measurements (e.g., isotope ages by Rb/Sr or Sm/Nd isochron dating
as in [1], ordering of the geological column as in [2] or [3],
and continental drift as in [4]).
[1] Dalrymple, G. Brent, 1991. _The Age of the Earth_. California:
Stanford University Press, ISBN 0-8047-1569-6. See: Tables
5.5 and 5.6 (pp. 230-231, 239) for lists of lunar rocks dated
by multiple isotopic methods; Table 6.3 (pp. 287-289) for lists
of meteorites dated by multiple methods.
[2] Harland, W. Brian, et al., 1990. _A geologic time scale 1989_.
Cambridge: Cambridge University Press, ISBN 0-521-38765-5.
The entire book details a large database of isotopic ages shows
their correlation with position in the geologic column.
[3] Dalrymple, G. Brent, 1986. "Radiometric Dating, Geologic Time,
and the Age of the Earth", U.S. Geologic Survey Open-File Report
86-110. See pp. 22-24 for a chart and discussion of correlation
between K-Ar age and stratigraphic position of the Land Mammal
Ages of the Cenozoic.
[4] Dalrymple, G. Brent, 1986. (Ref. [3]) See Figs. 2-3 and p. 25
for a chart of distance from the Hawaiian mantle plume versus
K-Ar age.
--
Chris Stassen http://www.stassen.com/chris/
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