Age spectra for microclines separated from samples obtained within the thermal aureole of the conduit to Obsidian Dome are compared with those from a reference sample collected in an area removed from the thermal effects of intrusion. The age spectra are characterized by a zero-age plateau at low fractional release and by an ~80 Ma plateau at higher fractional releases. These plateaus correspond to subparallel arrays in the Arrhenius data. These arrays are attributed to two discrete diffusion domain size fractions characterized by identical activation energies but by discrete and different frequency grain size fractions (Lovera et al., 1989). The relative losses of 40Ar* do not show a systematic pattern when compared on the basis of total 40Ar* loss. The 40Ar* losses first increase and then fall with distance from the contact. However, when the data composing the high-temperature array of the Arrhenius data are considered (the large diffusion domain size fraction), the pattern is consistent with that predicted by conductive heating of the aureole. This agreement requires the aureole to have uniformly preheated to a temperature of ~250¿ C prior to intrusion of the rhyolite dike and surrounding breccia funnel that make up the the vent structure. Further, the data require that the material forming the breccia funnel must have been heated to a temperature approaching that of the magma prior to emplacement. ¿ American Geophysical Union 1990