Question

What conditions are needed to use the Potassium-Argon Dating method?

Answer 1

Check this out: http://en.wikipedia.org/wiki/K%E2%80%93Ar_dating

Answer 2

Since Potassium-Argon and Argon-Argon dating techniques are the most common and are considered, even by geologists, to be among the most accurate of all the radioisotope dating methods, lets consider these in particular detail.
Argon is a noble gas. The main isotopes of argon in terrestrial systems are 40Ar (99.6%), 36Ar (0.337%), and 38Ar (0.063%). Naturally occurring 40K decays to stable 40Ar (11.2%) by electron capture and by positron emission, and decays to stable 40Ca (88.8%) by negatron emission; 40K has a half-life of 1.25 billion years.
Most of the argon isotope literature deals with measurement of 40Ar for use in 40K/40Ar dating of rocks. The conventional 40K/40Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., 40Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium, and the amount of radiogenic 40Ar that has accumulated. The minerals that are best suited for dating include biotite, muscovite, and plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered (Faure, 1986).
Under some circumstances the requirements for successful 40K/40Ar dating may be violated. For example, if 40Ar is lost by diffusion while the rock cooled, the age-dates represent the time elapsed since the rock cooled sufficiently for diffusive losses to be insignificant. Or, if excess 40Ar is present in the rock, the calculated age-dates are too old. The 40Ar/39Ar method is thought to be able to overcome this problem inherent with the 40K/40Ar method.
The 40Ar/39Ar dating method is based on the formation of 39Ar as a result of the intentional irradiation of K-bearing samples within a nuclear reactor. The bombardment produces various isotopes of Ar, K, Ca, and Cl, but the dominant source of 39Ar is from 39K. Radioactive 39Ar decays back to 39K by beta emission with a half-life of 269 years, but the decay is slow compared to the analysis time and can be ignored (Faure, 1986). The principal “advantage” of 40Ar/39Ar dating is that argon can be released partially by stepwise heating of irradiated samples, producing a spectrum of dates related to the “thermal history of the rock” (understanding that Argon is a gas while Potassium is not).
Because of this, it is much easier to determine a 40K/40Ar ratio and do it in a stepwise fashion with varying amounts of time and heat. This "stepwise" testing is thought to eliminate the errors caused by “extraneous” argon that might have “contaminated” the rock over time either by a loss or a gain of “outside” argon (ie: atmospheric argon). The problem with this theory is that who is to know which step, or average of steps in the process represents the “correct” 40K/40Ar ratio? How is this calibrated? Also, even if the argon-argon dating method does eliminate the "contamination" problem, it does not solve the problem of original argon. Did the clock get reset to zero when the volcano erupted? Or, was there some argon trapped in the rocks originally? Also, the 40Ar/39Ar dating method is not an independent dating method. It must be first calibrated against a sample of "known age". This age of this sample is usually determined by, you guessed it, the 40K/40Ar method (see discussion of Ar/Ar calibration below).
Recent experiments on volcanoes of known ages have been done using the 40Ar/39Ar dating method, which seem to confirm its accuracy. Recent testing of volcanic material from Mt. Vesuvius was dated accurately with the 40Ar/39Ar method to within seven years of the actual event.3 40Ar/39Ar Dating into the Historical Realm: Calibration Against Pliny the Younger was written by P. R. Renne et. al. and published in Science 277: 1279-1280 (1997). Renne tested Ar-Ar dating by checking it against the 79 A.D. eruption of Vesuvius that destroyed Pompeii. Renne and his team noted that, "Analysis of single crystals, for example by laser fusion, can obviate xenocrystic contamination, but single crystals are seldom large enough to yield measurable quantities of 40Ar* through radiogenic ingrowth in the Holocene [i.e. last 12,000 years]." Would Ar-Ar dating methods work such recent material? It apparently did. The testing returned an age of 1925±94 years. The true age was 1918 years. The test was off only 7 years. The conclusions of Renne and his team read as follows:

Thus despite the presence of excess 40Ar, a sample less than 2000 years old can be dated with better than 5% precision, validating 40Ar/39Ar dating as a reliable geochronometer into the late Holocene. These results also demonstrate that excess 40Ar can be identified in volcanic sanidine, and while perhaps negligible in pre-Holocene rocks, it has important consequences for sample at the limit of the method’s applicability. Further improvement in precision of 40Ar/39Ar analysis of historically dated samples may lead to welcome refinements in the ages of neutron fluence monitors, currently a limitation on the accuracy of the 40Ar/39Ar method. Our results also substantiate validity of the 40Ar/39Ar method in establishing the eruptive histories of populated active volcanic regions, where such information is vital to volcanic hazard assessment.

Of note however is that this test was not double blinded, and the number of such tests is not statistically significant as far as scientific analysis is concerned. Although interesting, it is basically a case study report, and as such it has very little scientific weight as far as statistical predictability.

Answer 3

@sikinder
How can this be summed up to a sentence or two? Like need the answer very briefly.