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Re: Uranium Dating & South African Sites?
A R Millard (A.R.Millard@durham.ac.uk)
13 Nov 1995 17:19:36 GMT
cc3265@CNSVAX.ALBANY.EDU wrote:
: Hi all!
: Can anyone tell me if Uranium series dating has ever been used on those
: troublesome South African sites, and, if not, why? I recently learned
: more about the procedures, that they work on breccia and (at least one
: isotope) has a half-life of 4.5by. So why wouldn't this be appropriate
: for South Africa? Last I read, they were using biostratigraphy to date
: the hominids. Is it because the sites are such a mess to begin with, &
: the Uranium series is so prone to error, that it is thought not to be
: worthwhile to try, or is there another, more technical, reason?
: I just know some student is going to be asking me this when we go over
: this material next week!
The problem with U-series dating of breccias is that they tend to consist
of old calcium carbonate in the form of limestone fragments of some great
age since formation and more recent precipitated calcium carbonate
cementing them together. If you can physically separate the two then you
can go ahead and date the cementing carbonate, the formation of which is
the event you are interested in, otherwise if you can't separate them you
will end up with a date which is some horrible mixture of the two and
useless.
Secondly the 4.5 billion yr isotope is U-238, which decays via two
relatively short lived intermediates to U-234 which in turn decays to
Th-230 (half life 75000 yr). This is the isotope usually used for
archaeological/palaeoanthropological dating, based on the fact that U is
soluble in water but Th is not, so when something like a breccia cement
precipitates it contains U but no Th and the build up of Th-230 from the
decay of U-234 is used for dating. The U-238 U-234 pair is not so useful
because the two isotopes are chemically virtually identical so they never
get separated. There are schemes for U-U dating, but they rely on having
some idea of the initial ratio of the two isotopes, which is only
possible in special circumstances (by the way they cover a range of up to
a few million years, being limited by the half-life of U-234).
Andrew Millard
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