From electron microprobe, petrographic and image analyses of samples from the northern tip of the Luzon Arc, the effects of various alteration reactions, phenocryst and groundmass assemblages on the whole-rock 40Ar/39Ar step-heating spectra have been delineated. The alteration of primary hornblende to an assemblage of zoisite + chlorite + sphene, the palagonitization of glass and the crystallization of phillipsite tends to cause discordant 40Ar/39Ar dates in the low-temperature steps of the age spectra. Discordant 40Ar/39Ar dates in the high-temperature steps appear to result from degassing of inherited or trapped 40Ar from coarse-grained phenocrysts. Samples in which most of the K resides in groundmass plagioclase tend to yield meaningful plateau dates, whereas samples in which most of the K resides primarily in palagonitized glass tend to yield meaningless plateau and isochron dates. Substantial loss of 39Ar (13-40%) occurred from irradiated samples prior to 40Ar/39Ar step-heating analysis, which appears to be correlated with the abundance of groundmass and the degree of alteration. This argon loss is attributed to thermally-induced argon diffusion from the fine-grained groundmass plagioclase, glass and alteration phases during the pre-step-heating bakeout.
