On the subject of argon dating, I think what is presented by AIG-ICR and other creationists stems partly from the fact that none of them have actually worked with the method and are basing their conclusions on perceived anomalies cribbed, cut and pasted from the scientific literature. The link you provided discusses K-Ar dating of which Ar-Ar is a variant. They then go on to discuss 'problems' with the method. First of all, no radiometric dating method is 100% accurate. As with all things in nature, there will be some rocks that simply cannot be dated. We have two choices, we can throw out all ages or we can examine each data point carefully to look for possible compromises to the assumptions we make when dating a rock. Creationists choose the former, science chooses the latter. If your watch fails to keep accurate time, do you get a new watch or throw out the whole notion of time?
At any rate, K-Ar dating always tells us something about the system we are dating, but the ages of older rocks tended to be more scattered about a general mean. Geologists realized that a variant of K-Ar dating could yield useful information about the incorporation of excess argon or the loss of argon into a mineral or rock. This method clearly showed why certain K-Ar ages varied. Ar-Ar basically looks at a step-by-step snapshot of the K-Ar systematics in a mineral. In the ideal case, each step (obtained by releasing small amounts of gas at increasing temperatures) should show the same age. This is called a plateau. In cartoon fashion it would look like this:
====================== 550 Ma
(each = is one release step and each release step gives an age of 550 Ma)
Note: Previewing this made me realize that the = signs below are not tabbed out. You'll have to imagine them stair-stepping up or down to the 550 Ma age)
In some cases, the crystal lattice will lose argon at very low temperatures and during the stepwise release of the we can see this as follows (same symbols)
=============== 550 Ma
= 540 Ma
= 530 Ma
= 520 Ma
= 510 Ma
= 500 Ma
In this sample, the argon loss is evident, but the crystal also retains a memory of the original age. The K-Ar age integrates all these steps into one, so the K-Ar age would lie somewhere between 500 and 550 Ma.
In other cases, excess argon might be incorporated into the lattice at the time of formation. The argon-argon system would the n look like this:
= 600 Ma
= 590 Ma
= 580 Ma
= 570 Ma
= 560 Ma
==================== 550 Ma
Here, the K-Ar age would yield something between 550-600 Ma whereas the Ar-Ar release pattern tells us that the K-Ar age is incorporating some extraneous argon. Scientists have developed these checks for a host of systems and examined the behavior of these isotopes during metamorphism (for example, K-Ar is easy to reset at low temoeratures whereas zircons are not). What is particularly amazing is how well these isotopic systems work and how consistent they are (see
http://www.clas.ufl.edu/users/jmeert/radiomet.htm ). There are a whole host of tricks employed by AIG-ICR to guarantee 'anomalous' looking ages, but in reality these are just tricks or lack of care during collection of the samples (for example
http://www.clas.ufl.edu/users/jmeert/crefaqs.htm ).
Cheers
Joe Meert
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