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 RbSr ages indistinguishable from UPb ages. Even though step heating plateaus were obtained for nearly a hundred 100 μm 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.

通过电子探针显微分析和⁴⁰ Ar / ³⁹ Ar测年分别筛选了博物馆级的Phalaborwa金云母（约2 Ga）和Rubikon伟晶岩（约0.5 Ga）大云母的晶内成分和时间异质性。已知两个云母的Rb Sr年龄与U都没有区别铅的年龄。即使为近百个100μm规模的亚晶粒碎片获得了逐步加热的平稳期，但在各个碎片之间也观察到百分之几的年龄变化。年龄变化与大晶体内的位置以及辐照罐中的位置无关。平行于和垂直于（001）平面的元素映射显示主要元素Ti，Fe，Mn，K，Al的浓度存在显着变化。阳离子迁移，元素再分布和逆向矿物形成无法通过视觉检测得以实现，因此需要开放系统的流体辅助化学反应。这些重结晶过程发生在一个或多个岩浆热液事件之后。亚晶粒年龄的空间分布表明⁴⁰ Ar / ³⁹Ar定年没有提供一个简单的，受扩散控制的冷却年龄，而是由矿物的倒退和重结晶控制的明显的年龄。结果是亚粒度范围内Ar的可变损失。这些观察表明，退行性事件不能通过高原年龄的存在或缺失来检测。取而代之的是，可以通过（1）通过基于微化学图的彻底的岩石学研究，以及（2）通过⁴⁰ Ar / ³⁹ Ar系统中的Cl / K和Ca / K约束来诊断重结晶。对于足够均匀的天然矿物以作为⁴⁰ K半衰期的精确校准品的要求仍然不令人满意。