An assessment of ⁴⁰Ar ³⁹Ar dating for the whole-rock volcanic samples from the Luzon Arc near Taiwan

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.