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Such failures may be due to laboratory errors (mistakes happen), unrecognized geologic factors (nature sometimes fools us), or misapplication of the techniques (no one is perfect).We scientists who measure isotope ages do not rely entirely on the error estimates and the self-checking features of age diagnostic diagrams to evaluate the accuracy of radiometric ages.Radioactive decay rates have been measured for over sixty years now for many of the decay clocks without any observed changes.And it has been close to a hundred years since the uranium-238 decay rate was first determined.The latest high-tech equipment permits reliable results to be obtained even with microscopic samples.Radiometric dating is self-checking, because the data (after certain preliminary calculations are made) are fitted to a straight line (an "isochron") by means of standard linear regression methods of statistics.
One question that sometimes arises here is how can scientists assume that rates of radioactivity have been constant over the great time spans involved.
Creationist Henry Morris, for example, criticizes this type of "uniformitarian" assumption [Morris2000, pg. But numerous experiments have been conducted to detect any change in radioactivity as a result of chemical activity, exceedingly high heat, pressure, or magnetic field. Scientists have also performed very exacting experiments to detect any change in the constants or laws of physics over time, but various lines of evidence indicate that these laws have been in force, essentially the same as we observe them today, over the multi-billion-year age of the universe.
None of these experiments has detected any significant deviation for any isotope used in geologic dating [Dalrymple1991, pg. Note, for instance, that light coming to Earth from distant stars (which in some cases emanated billions of years ago) reflects the same patterns of atomic spectra, based in the laws of quantum mechanics, that we see today.
Also, as the authors of the 1968 article were careful to explain, xenoliths cannot be dated by the K-Ar method because of excess argon in bubbles trapped inside [Dalrymple2006].
Thus in this case, as in many others that have been raised by skeptics of old-earth geology, the "anomaly" is more imaginary than real.