Dating Techniques

[smithjb@sas.upenn.edu (Joshua Smith)]

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From: "Allan Edels" <Edels@email.msn.com>
To: <dinosaur@usc.edu>
Subject: Re: Radiocarbon Dating

Brandon, etc.:

    The method that I remember (besides thermoluminescence), is the
measurement of the ratio of the radioactive material to its decay elements.
For example, Uranium (U-235 or U-238) runs into the Thorium series then
breakdowns into Radium and Radon, and finally, into Lead (the stable
isotope).

    If you find U-235 in your volcanic tuft, and you also find (stable) Lead
associated directly with it, you can work out how old the deposit is, by
comparing the amount of U-235 with the amount of Lead.  If the sample is 75%
U-235 and 15% Lead (and 10% other), then you know that the sample is
approximately 300 mya.  (This is a very rough and very crude estimate - done
off the top of my head).  About half of the half of the original amount (1/2
* 1/2 = 1/4) of U-235 has decayed into other materials - meaning that only
half of its half life has passed - therefore: ~300 mya.

    Josh Smith could probably give you more detailed and (more) correct
information, if he happens to be checking the list.  If I could find one of
my documents (which I seem to have buried somewhere), I could be slightly
more precise myself.  :-)

        Allan Edels


        I don't really have too too much to add to all of the discussion 
on techniques that has been flying around while I was gone.  The 
information being put out there is pretty much accurate.  I am not going 
to go into a large discourse or review of stuff, but I will be happy to 
answer specific questions if they are asked.  How is that for dodging work??

        The only thing that I will say again (I am starting to feel like 
a broken record) is that dating geologic units is a difficult, touchy, 
and potentially complicated task and that just because someone publishes 
a age for something doesn't mean that the age has anything to do with 
reality.  It is kind of like systematics--you need to know what your 
doing and take apart someones matrix character by character before you 
can assess how good the analysis is (yes, Tom, I DID just say something 
that wasn't completely trashing about cladistics...).  So too, you need 
to know what method someone used and more importantly, what the rocks are 
AND what their structural and stratigraphic relationships are.   



-- 
__________________________
Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
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