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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.