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In Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, La-Be, La-Ce and K-Ca dating, we check that the points we plot on the isochron diagram lie on a straight line.These precautions allow us to throw out most data that have been produced by confounding factors such as atmospheric contamination, weathering, hydrothermal events, metamorphism, metasomatism, etc.So in the U-Pb method, we check that the two uranium isotopes produce concordant dates.In the Ar-Ar method, we check that step heating yields the same date at every step.It is hard to think that this is a coincidence; it is also hard to think of any mechanism that could produce this agreement other than that the rocks are as old as radiometric methods tell us.
But it is equally far-fetched to imagine that three different mechanisms interfered with the three processes in such a way as to leave the dates concordant; that would require either a preposterous coincidence, or for natural processes to be actually conspiring to deceive us: an idea which is, if anything, even more preposterous. But in this case there is a perfectly reasonable and straightforward explanation for why the dates are concordant, namely that they are correct.Similar remarks may be made about the agreement between radiometric dating of rocks, sclerochronology, and dating by rhythmites.Are we to believe that one single mechanism interfered with the decay of radioactive isotopes, the secretion of calcium carbonate by molluscs, and the action of the tide? But are we instead to believe that three separate mechanisms interfered with these processes in such a way as to leave all the dates concordant? The straightforward explanation for the concordance of the dates is that they are in fact correct.It is hard to see this as a coincidence; it is equally hard to think of some alternate explanation of why we can correlate isotope ratios or sclerochronological data with the relative order of rocks as deduced from stratigraphic methods — other than the straightforward explanation that absolute dating is producing the right dates.In our discussion of radiometric dating, we have seen that many, indeed most, radiometric methods are self-checking.