G. W. Grindley: Have you found many examples of very high K–Ar ages in very young volcanic rocks and do you have any feasible physical explanations for the source and location of the high proportion of excess argon ?

Reply: The conventionally computed K–Ar or '°Ar/S=Ar ages of many young volcanic rocks can be shown to be discrepantly high when re-examined by the method described above. There may be numerous causes for this discrepancy, but three principal explanations appear to be: (a) the inclusion within juvenile minerals and rocks of a variable quantity of initial argon of unusually high 40Ar/39Ar ratio derived directly from the magma, (b) incomplete outgassing of accessory volcanic minerals and inclusions not readily distinguishable from the juvenile material on petrographic grounds alone and (c) the introduction ofextraneons argon of variable 40Ar/36Ar ratio into certain volcanic minerals during subsequent metasomatism.

R. L. Wilson : How is it that one has curves which fold back on themselves on the 40Ar/36Ar versus 39Ar/36Ar plots—'multivalued curves,' so to speak?

Reply: The 'folded back' nature of argon isotope ratio plots is a natural consequence of their derivation from step-wise degassing experiments. In the majority of samples analysed by this method, argon is released from locales of progressively greater potassium-content i.e. of progressively higher 39Ar/36Ar ratio, until a maximum value is reached, after which, at later steps, the 39Ar/36Ar ratio is progressively reduced because of the falling potassium-content of the higher temperature locales. As the 40Ar/36Ar ratio corresponding to any given

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