Abstract The presence of secondary phases (carbonates, zeolites, clay minerals, etc.) in volcanic rocks and subabyssal intrusive units (sills and dykes) often pose obstacles for high resolution 40Ar/39Ar geochronology as these phases may promote loss of radiogenic argon, production of interfering isotopes, or trapping of excessive atmospheric argon contents. Acid treatment is used to remove alteration products before sample irradiation and analysis. In an effort to assess and quantify the effects of alteration and the removal of alteration products by acid treatment on the accuracy and precision of 40Ar/39Ar analysis, three basalt samples from the Parana large igneous province previously dated in triplicate without acid treatment were dated again without and with acid treatment. Despite the presence of 14–28% clay minerals in the samples, which results in up to 67% atmospheric component in the total amount of 40Ar extracted from untreated grains, the presence of alteration phases and their removal by acid treatment have no significant role on the accuracy of 40Ar/39Ar step-heating ages. The only quantifiable effect is an ~0.1% increase in age precision for the acid-treated aliquots. The results reveal that effective separation of atmospheric and radiogenic gases in properly designed and sufficiently detailed step-heating experiments successfully obviates deleterious effects associated with some types of alteration phases, and it suffices to produce high-resolution ages. Therefore, acid treatment is a desirable but not essential step in high-resolution 40Ar/39Ar geochronology. Characterizing alteration phases, however, is an essential step as some secondary products (e.g., sericite, albite), if present, will not be leached during acid treatment and will significantly affect the accuracy of geochronological results either with or without acid treatment.

中文翻译：

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量化蚀变和酸处理对全岩玄武岩 40Ar/39Ar 年代学的影响

摘要 火山岩和海底侵入单元（基岩和岩脉）中第二相（碳酸盐、沸石、粘土矿物等）的存在通常对高分辨率 40Ar/39Ar 年代学构成障碍，因为这些相可能会促进放射性氩的损失，生产干扰同位素，或捕获过量的大气氩含量。酸处理用于在样品辐照和分析之前去除变质产物。为了评估和量化酸处理蚀变和蚀变产物去除对 40Ar/39Ar 分析准确度和精密度的影响，来自巴拉那大型火成岩省的三个玄武岩样品再次测年，以前未经酸处理一式三份没有和有酸处理。尽管样品中存在 14-28% 的粘土矿物，这导致未处理晶粒中提取的 40Ar 总量中高达 67% 的大气成分，蚀变相的存在及其酸处理去除对 40Ar/39Ar 阶梯加热年龄的准确性没有显着影响。唯一可量化的影响是酸处理等分试样的年龄精度提高了约 0.1%。结果表明，在适当设计和足够详细的阶梯加热实验中，大气和放射性气体的有效分离成功地消除了与某些类型的蚀变相相关的有害影响，并且足以产生高分辨率的年龄。因此，酸处理是高分辨率 40Ar/39Ar 年代学中理想但非必要的步骤。然而，表征蚀变阶段是一些次级产品（例如绢云母、