[ RadSafe ] 4.4 billion year old crystal

[Cary Renquist]

Heard the story on the radio this morning -- haven't had a chance to
read about it...
Critics of their earlier paper: while it was established that the zircon
didn't allow the uranium to move about, they did not have info on the
lead. In order to support their measurements, they developed a method
that allows them to track the individual atoms location within the
crystal. This allowed them to defend their original estimate of the 4.4
billion yr age based on the U/Pb ratios.


---
Cary Renquist
cary.renquist at ezag.com


-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Emer, Dudley
Sent: Monday, 24 February 2014 4:08 PM
To: The International Radiation Protection (Health Physics) Mailing List
Subject: Re: [ RadSafe ] 4.4 billion year old crystal

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"