Age plateaux and isochrons in the40Ar-39Ar and similar dating techniques can be severely altered by processes changing the geometric distribution of one isotope relative to the other. Age plateaux and isochrons can even be generated entirely as experimental artifacts. Alterations of40Ar-39Ar plateau ages by recoil redistribution of39Ar, incorporation of trapped40Ar and prior40Ar loss provide significant examples. 10% shifts in isotopic ratios are very easily obtained and would result in errors in40Ar-39Ar plateau ages of 4 AE old samples of ∼100 m.y., which is comparable to the age differences which must be resolved to develop early lunar and solar system chronology. The possible occurrence of diffusion artifacts must be evaluated in every case to establish that ages and age differences obtained by stepwise thermal release analyses are real. All studies involving the stepwise thermal extraction of multiple isotopic components may show similar diffusion artifacts. Constant isotopic compositions may be obtained during thermal release which do not represent the actual compositions of sample reservoirs. © 1976.
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