[ RadSafe ] 4.4 billion year old crystal

[Nick Tsurikov]

Dear Jeffrey,
In my understanding, it's not just the half-life.  If you'll have the time
-- please search for "metamict" zircon.  uranium atoms sit randomly inside
the crystal lattice (replacing zirconium atoms) and as they decay - there
is nowhere for Th-230, Ra-226, etc to go - it all stays inside.  Then,
naturally, the physical properties would change - depending on how much
uranium was there and how old the crystal is.
Plus, a typical zircon would have not only ~250 ppm U, but about 200 ppm Th
as well.
My guess is that the "degree of decay" could be determined with an
electronic microscope - this should allow people to see how many U atoms
have actually decayed.  Then the relationship between U and Pb would give a
rough age.
Of course, I could be all wrong...
Kind regards
Nick Tsurikov
Western Australia


On Tue, Feb 25, 2014 at 6:58 AM, Kulp, Jeffrey (DOH) <
Jeffrey.Kulp at doh.wa.gov> wrote:

> Hello Radsafers,
>
>
>
> I have a question about a news story I just read; it said that
> scientists in Australia had discovered a 4.4 billion year old zircon
> crystal.
>
>
>
> The article went on to say that they had dated it using uranium decay to
> lead.
>
>
>
> My question is how do they know what concentration of uranium the
> crystal started with? At present the concentration of uranium in the
> earth's                                                 crust varies
> quite a bit (two orders of magnitude or more?). Was the U concentration
> more uniform back when the crust just formed?
>
>
>
> Given that the half-life of U-238 is roughly 4.4 billion years, how can
> they be so sure of its age?
>
>
>
> Thanks in advance for the information
>
>
>
>
> Jeffrey Kulp, RRPT
>
> Radiation Health Physicist
>
> Washington State Department of Health - ORP
>
> 16201 E. Indiana Ave. Suite 1500
>
> Spokane Valley, WA 99216
>
> (509) 329-2138 (Office)
>
> (509) 329-2154 (Fax)
>
> "Public Health - Always Working for a Safer and Healthier Washington"
>
>
>
>
>
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