[ RadSafe ] 4.4 billion year old crystal

[Emer, Dudley]

Jeff,
They don't actually need the U-238 concentration as they look at the in-growth of Pb-206 from U-238 in a zircon crystal. The structure of Zircon (ZrSiO4) allows it to readily contain a uranium impurity but excludes a lead impurity. So once the zircon is formed (cooled and crystallizes) it starts the clock with all Uranium and no Pb.
The age is then determined by the Pb/U ratio. 

http://www.geology.wisc.edu/~valley/zircons/Wilde2001Nature.pdf

Dudley Emer
Geophysicist
National Security Technologies, LLC (NSTec)
National Center for Nuclear Security 
Nevada National Security Site, Mercury, Nv
U.S. DOE Contractor
702-295-7808 office
702-794-5824 pager
702-343-6208 cell
email: emerdf at nv.doe.gov
Mailing address:
P.O. Box 98521
M/S NNSS 980
Las Vegas, Nv 89193-8521

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu [mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Kulp, Jeffrey (DOH)
Sent: Monday, February 24, 2014 2:58 PM
To: radsafe at health.phys.iit.edu
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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