The K-Ar and Ar-Ar methods are the most reliable sources of radiometric dating. The K-Ar method is the only decay scheme that can be used with little or no concern for the initial presence of the daughter isotope. This is because 40Ar is an inert gas that does not combine chemically with any other element and so escapes easily from rocks when they are heated. Thus, while a rock is molten, the 40Ar formed by the decay of 40K escapes from the liquid.2 (Dalrymple)
And These
the most, (some say only), reliable sources are flawed. The igneous rock that is found within layers of the earth is what is tested
the methods most of you already know
no need to type that out
heres the catch
the initial presence of Ar already in the rock. Here is a little bit of what Andrew Snelling, an associate professor of geology in El Cajon, California had to say.
According to the assumptions foundational to potassium-argon (K-Ar) and argon-argon (Ar-Ar) dating of rocks, there should not be any daughter radiogenic argon (40Ar*) in rocks when they form. When measured, all 40Ar* in a rock is assumed to have been produced by in situ radioactive decay of 40K within the rock since it formed. However, it is well established that volcanic rocks (e.g. basalt) contain excess 40Ar*, that is, 40Ar which cannot be attributed to either atmospheric contamination or in situ radioactive decay of 40K. This excess 40Ar* represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.
However, are all other rocks in the earth's crust also susceptible to "contamination" by excess 40Ar* emanating from the mantle? If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable.
When muscovite (a common mineral in crustal rocks) is heated to 740°-860°C under high Ar pressures for periods of 3 to 10.5 hours it absorbs significant quantities of Ar, producing K-Ar "ages" of up to 5 billion years, and the absorbed Ar is indistinguishable from radiogenic argon (40Ar*).2 In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0.5 wt% Ar at 640°C and a vapor pressure of 4,000 atmospheres.3 This is approximately 2,500 times as much Ar as is found in natural muscovite. Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize.
Because it is known that excess 40Ar* is carried from the mantle by plumes of mafic magmas up into the earth's crust, it is equally likely that much of the excess 40Ar* in crustal rocks could be primordial 40Ar. Thus, we have no way of knowing if any of the 40Ar* measured in crustal rocks has any age significance. Additional to the primordial 40Ar from the mantle is 40Ar* released from minerals and rocks during diagenesis and metamorphism, so that there is continual migration and circulation of both primordial 40Ar and 40Ar* in the crust which is reflected in their presence in CO2-rich natural gases. Therefore, when samples of crustal rocks are analyzed for K-Ar andAr-Ar "dating," one can never be sure that whatever 40Ar* is in the rocks is from in situ radioactive decay of 40K since their formation, or if some or all of it came from the mantle or from other crustal rocks and minerals. Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them.
In respect to the many test on rocks formed from lava flows
evolutionists say that since argon is a gas, all of it should have escaped from the lava before it cooled. Therefore, all the 40Ar in the rock should be the result of decay from potassium. Based on the measured potassium, argon, and the decay rate, they calculate an age. That is why it does not matter how long the magma was in the volcano before it erupted. They believe that when the volcano erupts, all the 40Ar escapes, and the atomic clock gets reset to zero.
If all the argon escaped from hot lava of volcanoes that erupted long ago, then all the argon should escape from the hot lava of volcanoes that erupt in modern times too. But modern lava does have 40Ar in it. This is known as the excess argon problem in geological circles. My position is that there is no such thing as excess argon. The rocks have the right amount of argon in them. This amount just happens to be more than the amount predicted by an incorrect theory. (Sean D. Pitman M.D.)
This is argon that cannot be atmospheric or decayed from K
Funkhouser and Naughton found that the excess 40Ar in the 1800-1801 Hualalai flow, Hawaii, resided in fluid and gaseous inclusions in olivine, plagioclase, and pyroxene in ultramafic xenoliths in the basalt, and was sufficient to yield "ages" of 2.6 Ma to 2960 Ma. (Sean D. Pitman M.D.) Since these materials come from the mantle
the Ar must initially be there.
Many recent studies confirm the mantle source of excess 40Ar. Hawaiian volcanism is typically cited as resulting from a mantle plume, most investigators now conceding that excess 40Ar in the lavas, including those from the active Loihi and Kilauea volcanoes, is indicative of the mantle source area from which the magmas came. Considerable excess 40Ar measured in ultramafic mantle xenoliths from Kerguelen Archipelago in the southern Indian Ocean likewise is regarded as the mantle source signature of hotspot volcanism.14 Indeed, data from single vesicles in mid-ocean ridge basalt samples dredged from the North Atlantic suggest the excess 40Ar in the upper mantle may be almost double previous estimates, that is, almost 150 times more than the atmospheric content (relative to 36Ar).15 Another study on the same samples indicates the upper mantle content of 40Ar could be even ten times higher.16
Further confirmation comes from diamonds, which form in the mantle and are carried by explosive volcanism into the upper crust and to the surface. When Zashu et al. obtained a K-Ar isochron "age" of 6.0±0.3 Ga for 10 Zaire diamonds, it was obvious excess 40Ar was responsible, because the diamonds could not be older than the earth itself.14 These same diamonds produced 40Ar/39Ar "age" spectra yielding a ~5.7 Ga isochron.17 It was concluded that the 40Ar is an excess component which has no age significance and is found in tiny inclusions of mantle-derived fluid. (Sean D. Pitman M.D.)
So we see that pressure can influence the initial presence of daughter isotopes. And, we see results from many test recording daughter isotopes initially being in the rocks from formation. Argon is being produced in the earths crust all of the time
Potassium is about 2.5% of the earth's crust. About 1/10,000 of potassium is 40K, which decays into 40Ar with a half-life of 1.25 billion years. Actually, only about 1/10th of the40K decays to Argon, and the rest decays to calcium. Argon is about 3.6 x 10-4 % of the earth's crust. We can assume then that the magma is probably about 2.5% potassium and about 0.00025% of the radioactive form, Potassium-40 (40K). Now, Lets say we are trying to date a one billion year old rock. How much of it would be 40K? Starting with 0.00025% as the modern concentration of 40K in magma, we would have to divide by roughly two (About one half-life). This would leave us with a 0.000125% of 40K. Now, about 90% of the decay product is calcium and only about 10% is Ar-40. This gives about 0.0000125% 40Ar in the total make-up of the rock. This is about one ten millionth of the mass of the rock, a very tiny fraction. If the rock weighed one gram, the Ar-40 in the rock would weight one ten millionth of a gram. And yet, with a relatively large amount of argon in the air, argon filtering up from rocks below, excess argon in lava, the fact that argon and potassium are water soluble, and the fact that argon is mobile in rock and is a gas, we are still expecting this wisp of argon gas to tell us how old the rock is? The percentage of 40Ar is even less for younger rocks. For example, it would be about one part in 100 million for rocks in the vicinity of 50-60 million years old. However, to get just one part in 10 million of argon in a rock in a thousand years, we would only need to get one part in 10 billion entering the rock each year. This would be less than one part in a trillion entering the rock each day, on the average. This would suffice to give a rock an average computed potassium-argon age of over a billion years. (Sean D. Pitman)
However, leaching also occurs :
Leaching also occurs, releasing argon from rocks. Heating of rocks can also release argon. Argon is released from lava as it cools, and probably filters up into the crust from the magma below, along with helium and other radioactive decay products. All of this argon is being produced and entering the air and water in between the rocks, and gradually filtering up to the atmosphere. But, we know that some minerals absorb argon (correction factors are applied for this when using K-Ar dating). So this argon that is being produced will leave some rocks and enter others. The various pressures, temperatures, moisture, nature of the materials and a variety of other factors all play together to challenge the validity of K-Ar and/or Ar-Ar dating.(Sean D. Pitman M.D.)
this is just the beginning
the other methods have even more flaws
this is considered the best one.
scientist 
