⁴⁰Ar/³⁹Ar geochronology is one of the most important techniques for constraining the timing of basaltic events due to the paucity of suitable minerals in basalts for other geochronological techniques such as U–Pb (e.g., zircon, baddeleyite). Among a variety of materials from basaltic rocks that have been used for ⁴⁰Ar/³⁹Ar dating, plagioclase is the most important due to its common presence in basalts as a primary crystallizing phase, and its transparency so that fresh grains can be selected during sample preparation. However, plagioclase ⁴⁰Ar/³⁹Ar geochronology has often been compromised by alteration (e.g., sericitization by hydrothermal events), which, in practice, is difficult to identify using a petrographic microscope when the amount of alteration is low (e.g., < 1%).

We used laboratory step-heating experiments and theoretical simulations to characterize the ⁴⁰Ar/³⁹Ar age and Ca/K spectra of altered plagioclase so that ⁴⁰Ar/³⁹Ar dating results on altered samples can be identified and better interpreted. The step-heating experiments and theoretical simulations yielded consistent results, and show that with the presence of even a tiny amount of sericite (∼0.01% for K-poor samples and 0.1% for K-rich samples), the plagioclase samples yielded alteration plateau ages that are 3%–4% younger than the crystallization age. The difference between the alteration age of sericitized plagioclase and its crystallization age is primarily controlled by the time lapse between the crystallization and sericitization events, but also by the Ca/K ratios of the plagioclase. For plagioclase samples that experienced the same alteration event, the higher the Ca/K ratio is, the more sensitive the ⁴⁰Ar/³⁹Ar age is to alteration. We propose that the alteration signatures of plagioclase can be effectively identified through inspecting the ⁴⁰Ar/³⁹Ar age spectra, the Ca/K spectra, and the degassing curves. We also investigated the effect of sericitization of plagioclase microliths in basaltic groundmass and modelled the ⁴⁰Ar/³⁹Ar age and Ca/K spectra of altered groundmass samples. We validate our approach by revisiting published ⁴⁰Ar/³⁹Ar dating results for large igneous provinces, and showed that these dates should have been interpreted as alteration ages (minimum eruption ages) rather than crystallization ages. Finally, we demonstrate that with high degrees of alteration (∼50% for K-poor and > 70% for K-rich plagioclase samples), the age of hydrothermal alteration can be successfully dated.

⁴⁰ Ar/ ³⁹ Ar 年代学是限制玄武岩事件发生时间的最重要技术之一，因为玄武岩中缺乏适合其他年代学技术如 U-Pb（例如锆石、斜斜锆石）的矿物。在用于⁴⁰ Ar/ ³⁹ Ar 测年的各种玄武岩材料中，斜长石是最重要的，因为它作为主要结晶相普遍存在于玄武岩中，并且它的透明度使得在取样时可以选择新鲜颗粒准备。然而，斜长石⁴⁰ Ar/ ³⁹Ar地质年代学经常受到蚀变（例如，热液事件引起的绢云母化）的影响，实际上，当蚀变量较低（例如，< 1％）时，很难使用岩相显微镜进行识别。

我们使用实验室阶梯加热实验和理论模拟来表征改变斜长石的⁴⁰ Ar/ ³⁹ Ar 年龄和 Ca/K 光谱，因此⁴⁰ Ar/ ³⁹可以识别和更好地解释改变样本的 Ar 测年结果。逐步加热实验和理论模拟产生了一致的结果，并表明即使存在少量绢云母（贫钾样品为 0.01%，富钾样品为 0.1%），斜长石样品也会产生蚀变平台年龄比结晶年龄年轻 3%–4%。绢云母化斜长石的蚀变年龄与其结晶年龄之间的差异主要受结晶和绢云母化事件之间的时间间隔控制，但也受斜长石的 Ca/K 比率控制。对于经历相同蚀变事件的斜长石样品，Ca/K 比值越高，⁴⁰ Ar/ ³⁹Ar 年龄是改变。我们建议通过检查⁴⁰ Ar/ ³⁹ Ar 年龄谱、Ca/K 谱和脱气曲线可以有效识别斜长石的蚀变特征。我们还研究了玄武岩基质中斜长石微石化的影响，并对改变的基质样品的⁴⁰ Ar/ ³⁹ Ar 年龄和 Ca/K 光谱进行了建模。我们通过重新审视已发表的⁴⁰ Ar/ ^{39 来}验证我们的方法大型火成岩省的 Ar 测年结果，并表明这些日期应该被解释为蚀变年龄（最小喷发年龄）而不是结晶年龄。最后，我们证明了高程度的蚀变（贫钾斜长石样品为~50%，富钾斜长石样品> 70%），可以成功地确定热液蚀变的年龄。