Quick radiometric dating question- misused techniques

For the specific case of K-Ar, the method is known to give erroneous dates if the lava cooled too quickly. The 39Ar-40Ar method was invented to correct for this, and has been shown to be much more accurate and reliable. K-Ar dates are much less expensive, but should be taken with a grain of salt until they are verified by 39Ar-40Ar or other means.Isotopic dating is subject to many variables and pitfalls. The more that is known about a sample, the better it can be prepared and dated. Thus, blind samples are generally frowned upon, and some labs will refuse to accept them.

You'd have to see the actual lab report to see what was claimed in this case. Was the measured amount of argon consistent with measurement background? If so, the result should have said "less than" a background date. If not, the result should have indicated a date range. The way to measure excess argon is by the 39Ar-40Ar method. (Also called the Ar-Ar method.)

FYI, here's a simplified description of how the 39Ar-40Ar method works:Basic K-Ar dating relies on the decay of 40K to 40Ar in formerly molten rock. Assuming that all of the original argon diffused out while the rock was molten, the present amount of argon can be measured (by heating the sample and allowing the Ar to diffuse out) and the age since the rock was molten can be calculated.If the rock cooled too fast or cooled under pressure (e.g. underwater lava flows), argon can be trapped in the rock. It is also possible for argon to diffuse into the rock over time. In either case, K-Ar dating will give an incorrect result due to the excess argon.But there is a physical difference in the location of this excess argon and the argon resulting from radioactive decay. As the mineral grains crystalize, any initial argon will be forced out of the grain bulk and to the grain boundaries. Likewise, diffusion rates are much higher along grain boundaries than through the grain bulk, so any later diffusion will primarily add argon at grain boundaries. In contrast, radioactive decay will generate argon in the grain bulk.So we need some way to distinguish where the argon resides (grain boundaries or bulk), and this is what the 39Ar-40Ar method provides.The 39Ar-40Ar method first neutron-activates 39K to 39Ar in a nuclear reactor. This creates 39Ar where the 39K was, i.e. in the mineral grain bulk, not on the grain boundaries. Now the sample is heated in a mass spectrometer and both the 39Ar and 40Ar are measured. The initial 40Ar/39Ar ratio consists of argon diffusing from grain boundaries so does not reflect the true age. As time progresses, argon will start to diffuse from the bulk and the ratio will stabilize at a value reflecting the true age.The parameters that are needed for the age calculation are:
1) a good characterization of the neutron dose in the reactor (often done with concurrent calibration samples of known age)
2) knowledge of the original or current 39K/40K ratio in the rock