[ RadSafe ] 4.4 billion year old crystal

Kulp, Jeffrey (DOH) Jeffrey.Kulp at DOH.WA.GOV
Mon Feb 24 16:58:07 CST 2014

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


The article went on to say that they had dated it using uranium decay to


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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