Richard Wein (tich@primex.co.uk) wrote in reply to my post:
>>Since the average life expectancy of any radiometric isotope is on the
>>order of 10 times the half-life, we can reasonably infer that the
>>elements of which our earth is composed were created no less than
>>about 1-7 billion years ago.
>
>I don't know anything about radiometric dating, but this sounds odd to me.
>What do you mean by the "average life expectancy" of an isotope? After 10
>half-lives there should still be one thousandth of the original material
>left. Is this too small to detect?
Richard,
Thanks for the opportunity to clarify. I wrote that last paragraph in a
hurry before rushing off for an appointment and didn't review it well. As a
rule of thumb, we generally cannot detect enough of a radiometric isotope
after about 10 half-lifes to give a valid date. On a logrithmic curve, the
amount of remaining isotope approaches infinity, however in reality you
eventually decay the last atom. With better detection equipment, we may be
able to see that last atom. BUT, since the decay is a random process and
measurements are valid only on populations of atoms, any assigned dates
based on the last few atoms would have very large error bars.
> And anyway, your argument would give an *upper* limit of 7 BY.
>
>Shouldn't you be looking at the half-life of Sm-146 (70 MY), the longest
>lasting of the elements that *can't* be found? If the Earth is about 4.5 BY
>old, this is 64 half-lives, so only about 10^ -20 of the original material
>should be left. It sounds plausible that that would be too small to detect.
From the table of data that I gave and using 10 half-lifes as a rule of
thumb, the range of possible limits for the creation of elements found in
our solar system would fall between a lower limit of 700 M.Y. and an upper
limit of 7 B.Y.
In my original post, I rounded that lower limit a little to let the last
atom decay.
As you noted, the current estimate of the Earth's age is about 4.5 B.Y.
Last time I heard, the age of the solar system is estimated at about 5 B.Y.
Those numbers fall between the 0.7 & 7 B.Y. range predicted by the spread of
naturally occurring isotopes. Amazing coincidence, isn't it?
>
>Have I missed something?
>
Just clarity in my original post.
Steve
[The opinions expressed herein are my own
and are not to be attributed to my employer.]
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Steven M. Smith, Geologist Office: (303)236-1192
U.S. Geological Survey Fax: (303)236-3200
Box 25046, M.S. 973, DFC smsmith@usgs.gov
Denver, CO 80225
--The USGS National Geochemical Database NURE HSSR Data Web Site--
http://greenwood.cr.usgs.gov/pub/open-file-reports/ofr-97-0492/
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