We have discovered a curious ''fossilized'' asymmetric radiogenic argon wave trapped in single grains of biotite in the contact zones between a diabase dike (≈570 Ma) and the Grenville country rock (tonalitic gneiss ≈1000 Ma) near Mattawa, Ontario, Canada. The phenomenon is presumably the result of the interplay between the partial pressure of the ambient radiogenic argon and the opening and closing (blocking) of biotite to argon movement during the period of thermal (and fluid?) perturbation following the dike intrusion. The wavelength is roughly equal to one dike width (≈30 m).

The only previously reported example of such a wave (which we call an ''argonami'') was one discovered in 1970 by Wanless and his collegues in a study of biotites from a transverse across the Grenville Front. The two argonamis are remarkably similar, even in their asymmetry, but the wavelength scales differ by roughly two order of magnitude, presumably reflecting the relative sizes of the Mattawa dike and the continental-scale Grenville Front. The occurrence of the argonami phenomenon suggests that in some instances, the analysis of excess argon may become a useful tool for the unraveling of complex tectonothermal and fluid-migratory events taking place at plate boundaries and igneous contacts. ¿American Geophysical Union 1993