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Re: info wanted



C14 is not really good much beyond 50-60,000 years BP, and then only using 
accelerated mass spec methods.  

Potassium argon is the older of the argon methods.  Argon39/40 is a 
relatively new technique that requires the sensitivity of the new mass 
spectrometers.  The argon/argon method is a much more refined dating 
technique, with a tight age range and small error bars.

U series dating is indeed used for Cenozoic rocks.  You may be thinking of 
uranium-lead, but the U-series is quite sensitive in young rocks.  

Another isotopic method is strontium isotopes, Sr86/87.  I think for more 
accurate information you need the input of an isotopic geochemist.  

By the way, a geochronologist corrected me on my inaccurate use of the term 
radiometric-which is only used for C14, with a radiometer.  Other dating 
methods are isotopic, using various mass spectrometers.