[ RadSafe ] 4.4 billion year old crystal

[dlawrencenewyork at aol.com]

Jeffery,


This has to do with the fact that they were dating a Zircon crystal which when formed has  essentially no lead in its' crystalline structure but will incorporate Uranium (and Thorium); therefore any lead incorporated into it can inferred to be the result of radioactive decay. In a closed system (eg no lead loss) you can use the combined Pb-206/U-238 and Pb-207/U-235 ratios to fairly accurately date the crystal out to about 4.5 billion years assuming your crystal is in good shape. This works for some other crystals also, but I think not as well since the Zircon is very resistant to weathering and incorporates absolutely no lead when formed.


Best Regards,
David



-----Original Message-----
From: Kulp, Jeffrey (DOH) (DOH) <Jeffrey.Kulp at DOH.WA.GOV>
To: radsafe <radsafe at health.phys.iit.edu>
Sent: Mon, Feb 24, 2014 5:58 pm
Subject: [ RadSafe ] 4.4 billion year old crystal


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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