Potassium-Argon Dating

I'm new to this particular forum, but I've been on some others before. My question is for any geologists out there. I am a chemistry graduate student, so I should be able to handle any amount of detail you're willing to throw at me; technical information is welcome.On what assumptions or conditions does K-Ar dating rest, other than:* that the rock being dated has not undergone a significant heating/cooling cycle since it was initially formed
* that the rock is impermeable to Ar, and so traps what Ar-40 is a result of radioactive decay and does not leach Ar-40 from the atmosphere
* that the rock is sufficently old for some buildup of Ar-40 (100K years or so)I'm a bit confused because Geochron labs has their minimum date on the order of 0.5M years, while some webpages I'm finding at UCSB talk about K-Ar dating possibly being useful for dating rocks as young as 20K years.I've skimmed some reports by creation "scientists" such as Steve Austin describing a mockery of this technique by using it to derive dates of up to a million years for historic lava flows. I use the words "up to a million" advisedly, because all of the supposedly erroneous dates fall nicely at the bottom end of the spectrum K-Ar is meant to test.I also saw a defense of those techniques by some guy at AIG, but I didn't like his assertion that the error of the measurements being 60K years and the mean being ~300K years strongly indicated that it was the test itself, not the experiment, that was the problem. Is there some natural distribution to the amount of Argon present in rocks, such that we can measure the amount actually there to a finer precision than the natural variation? If so, how does this distribution arise? If not, why do we not meaure zero Ar-40 in rocks that were just coughed out of a volcano?If possible, I'd like to design a quiz question for my freshman chemistry students on K-Ar dating, so I'm trying to know as much background as possible.Thanks very much for your comments,Biophysicist

Thanks! That was a really informative website. Particularly interesting was the method by which we can get a measurement of the initial parent/daughter ratios through use of different minerals in a single rock in methods such as Rubidium dating.Do I surmise correctly that you don't need to know the propensity of Rubidium and Strontium to ihabit the different minerals, but merely isolate separate crystals from the rock and measure the abundances of each element?Weins is from my hometown. He's a welcome antidote to John Baumgardner, although I didn't know about him when I was growing up there.Biophysicist

Thanks Sylas. I gleaned a number of your points from reading the previous post's suggested website. Very good information, although I did catch a couple of lexical errors.I'm all for having a meaty discussion on some issue of science, and I appreciate your admonition to be careful about making assertions.On another forum, more than a year ago, I was a bit of a jerk. I logged on a pretended for weeks to be a creationist, making a number of really asinine statements before the "evolutionists" finally called me on it. Then, a few weeks later, I turned right around and did it again under a different name. After I gave myself away, I think a number of the creationists were a bit sore. I won't be doing that here.I must ask, though. I've noticed moderated debates with "evaluations" of each side's performance. Are the moderators truly "fair and balanced" or are they like Fox News (something you Aussies haven't had to deal with first-hand)?Biophysicist