@Wuwei doesn't know about C14 dating, how creationists used C14 dating, nor can explain why soft tissue and C14 remained in billions of years old dinosaur fossils. He didn't know it's not used to date rocks. His basic argument is from wikipedia and his atheist scientists cannot possibly know how much K was there initially.
For long live dating using potassium (K-40)
the original amount of potassium is the sum of the argon (Ar-40) and K-40 measured in moles.
For example
if it is found that there is an equal amount of K-40 and Ar-40 in a sample
the total potassium that started out is the sum of the two isotopes. Since the half life of K-40 is 1.28 billion years, the age must be the same, namely 1.28 billion years.
To verify the sample is not contaminated by environmental argon, there should not be any of the other naturally occurring isotopes of argon such as Ar-36 or Ar-38.
In using Uranium-238 to Lead-206 dating, contamination is eliminated by using U-238 encapsulated in zircon crystals. Contamination by natural occurring lead is verified by noting there should not be any other stable lead isotope occurring in the same sample.
Meteorites and very old earth rocks have been dated by many methods which agree with each other: rubidium-strontium; samarium-neodymium; lutetium-hafnium, uranium-lead and its variant Pb-Pb.
You have no case with thousands of assays that all agree with each other. Contamination is a random process when considering many wide spread samples sites.
I have directly dealt with isotopes and instruments for measuring radiation for over 25 years. You have not, and really show your naiveite.