Re: [asa] radiometric question

Bernie,
  I think we need to clarify what radiometric dating does. It doesn't really date a rock. It dates the thermal history, or more specifically, the time of a particular stage of its thermal history. As a result, different parts of a rock could have a different apparent age if it didn't cool uniformly. Or if a rock with high temperature stability is embedded in a lower melting point matrix and it all crystallizes, you'll have different results from different regions.

Therefore it isn't right to say that you have to know its age before you measure its age. But you do have to know something about thermal histories and the various environments which can set or reset its thermal (radioactive) clocks. A good example is the helium in zircon case which we discussed a year or two ago when talking about the RATE project. The helium concentration allows one to measure the time back to the closure temperature, or the time at which the rock had cooled to the point where helium no longer diffused out of the rock. A uranium based radioactive measurement would indicate when the temperature locked in the uranium or its byproducts. There are usually checks and balances so one can determine with reasonable assurance which technique is valid for which rocks. Some rocks simply can't be dated reliably.

So different techniques reveal different aspects of the history. As for the "correct recent date", it depends on what you were looking for. If you are looking for the date of a recent volcanic eruption, there's no simple way I know of that you could get that from potassium-argon.

Randy

Bernie wrote:
  Let me ask a clarifying question. Suppose a YEC takes a rock from Mt. St. Helen's and asks for a date due to radiometric dating. I assume various dating methods will be used. will they get the correct recent date? I didn't see a clear, blunt, answer.
   

  .Bernie

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Received on Wed Nov 5 16:58:24 2008