Thanks for answering.
OK, I follow. Actually, I pretty much understood this already. The part I am stumped on is the difference between rock and living matter. With living matter we can know pretty well when it died because we are measuring C14 from the carbon it ate - when it died, it stopped eating.
Rocks don't eat. They do get formed, but they get formed out of something else. If I followed the story correctly, they were claiming that all the rocks landed on the moon at about the same time because they were all aged the same. How does that get proven with K40 (or whatever) dating?
It seems to me that all we know is that the original matter that the rocks came from was formed based on the age. But does that prove those rocks were formed then, or just the planet? Wouldn't whatever the rock comes from have already started decaying? Would a newly formed rock have the already aged mix?
That's my problem with understanding. Rocks don't eat. They are just combinations of earlier things.
As for aging the craters based on the age of the rocks, that seems beyond foolishness to me. Who knows how long some of those rocks were floating around before they intersected.
Anyway, thanks for the answers. Hopefully there is one more left.
No, I am joking about the Holiday Inn, but the limit of my geology knowledge is one class that I took for fun a couple of years ago. I did, however, have a good grasp for the concepts. I hope though that if I accidentally mislead you that there will be a real geologist along to smack me upside the head.
Anyway, if I follow the story correctly, the scientists are not claiming that the dated rocks were those that impacted the moon (as you said, that would be foolish). I believe they are following the theory that the moon "seas" were created by collisions big enough to create a break in the moons crust through which magma flowed and then solidified, creating newer, darker rock on the surface. Since the moon mission that gathered the rocks landed in one of these areas, that would be the type of rocks they would be collecting. Also, since there is apparently no plate tectonic activity on the moon, the only way for new rock to be formed would be through these collisions. Note that according to the article, other scientists make a point that the rock samples were not collected from enough sites to warrant a generalization of there having been one huge cataclysmic event.
Back to the isotopes (and keep in mind that my example isotope, though a common one, may not have been the one the scientists were measuring). Potassium is a very common element in igneous rocks (a common example is potassium feldspar - the creamy pink mineral found in some granite). Argon, on the other hand is a noble gas, rarely forming any compounds. So, I am thinking that most of any argon in the magma at the time of rock formation would escaped as gas. So, the argon found in the rock samples would have been created by the radioactive decay of the K-40 after the rock had solidified and would be trapped in the rock structure.
That is about all I can come up with. Hopefully this helps somewhat. If not, I hope a geologist will wander by and enlighten us.