Subgrain 40ar/39ar Dating Of Museum-Quality Micas Reveals Intragrain Heterogeneity

Museum-grade mica megacrysts of the Phalaborwa phlogopite (ca. 2 Ga) and the Rubikon pegmatite (ca. 0.5 Ga) were screened for intra-grain compositional and chronological heterogeneities by electron probe microanalysis and 40Ar/39Ar dating, respectively. Both micas were known to have Rb?Sr ages indistinguishable from U?Pb ages. Even though step heating plateaus were obtained for nearly a hundred 100 ?m scale subgrain chips, age variations of several percents were observed among individual chips. The age variations were unrelated to the position within the megacryst and to the position in the irradiation canister. Element mapping parallel and perpendicular to the (001) plane showed significant variations in concentrations of major elements Ti, Fe, Mn, K, Al. Cation transport, element redistribution and retrograde mineral formation that escaped visual detection require open-system, fluid-assisted chemical reactions. These recrystallization processes occurred during one or several post-magmatic hydrothermal event(s). The spatial distribution of sub-grain ages show that 40Ar/39Ar dating does not provide a simple, diffusion-controlled cooling age, but rather apparent ages controlled by mineral retrogression and recrystallization. The result was variable loss of Ar at the subgrain scale. These observations show that retrogression events cannot be detected by the presence or absence of plateau ages. Instead, recrystallization can be diagnosed (1) by a thorough petrological investigation based on microchemical maps, and (2) by Cl/K and Ca/K constraints from 40Ar/39Ar systematics. The quest for a natural mineral sufficiently homogeneous to act as a precise calibrator for the 40K half-life remains unsatisfactory.