Abstract Radiokrypton dating using the long-lived natural isotope 81Kr has been developed for determining the age of ancient groundwater. 81Kr is attractive for this purpose because it is generally thought to be produced solely in the upper atmosphere. Its 229,000-year half-life, a spatially homogeneous distribution in the atmosphere, and the absence of anthropogenic sources makes it an ideal tracer to determine ages up to ∼1.3 million years. As a noble gas, it is inert and thus not subject to interfering geochemical reactions that may alter the age information. We sought to date groundwater samples collected from deep (0.6–1.9 km) rock fractures in the Kaapvaal Craton, South Africa. Previous studies using other dating methods have estimated groundwater ages at these sites in the 1–100 million year range. Surprisingly, three of the four samples collected from flowing boreholes in gold and diamond mines showed 81Kr isotopic abundances at 2–5 times the atmospheric value. This is the first time that underground production of 81Kr has been detected, indicating that 81Kr can be generated underground in measurable quantities in contrast to the long-held paradigm that such production is insignificant in natural rocks. A radionuclide production and release model is proposed in order to quantify the importance of different factors that affect the concentrations of 81Kr in the groundwater. It is not only the high effective uranium content of the rock but also a higher 81Kr fission yield than previously anticipated that are likely causing the elevated 81Kr values in this case. In less extreme environments with average crustal composition (i.e. moderate U concentration), however, we anticipate that the underground production minimally affects groundwater 81Kr dating as demonstrated.

中文翻译：

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在地下流体中检测到 81Kr 的地下生产

摘要 使用长寿命天然同位素 81Kr 进行放射性氪定年已被开发用于确定古代地下水的年龄。81Kr 对此很有吸引力，因为它通常被认为仅在高层大气中生产。其 229,000 年的半衰期、在大气中的空间均匀分布以及不存在人为来源，使其成为确定约 130 万年年龄的理想示踪剂。作为一种惰性气体，它是惰性的，因此不会受到可能改变年龄信息的干扰地球化学反应的影响。我们试图确定从南非 Kaapvaal Craton 的深层（0.6-1.9 公里）岩石裂缝中收集的地下水样本的年代。先前使用其他测年方法的研究估计这些地点的地下水年龄在 1-1 亿年范围内。出奇，从金矿和钻石矿的流动钻孔中收集的四个样本中有三个显示 81Kr 同位素丰度是大气值的 2-5 倍。这是首次探测到 81Kr 的地下生产，这表明 81Kr 可以以可测量的数量在地下生产，这与长期以来认为这种生产在天然岩石中微不足道的范式相反。提出了一种放射性核素产生和释放模型，以量化影响地下水中 81Kr 浓度的不同因素的重要性。在这种情况下，可能导致 81Kr 值升高的不仅是岩石的高有效铀含量，还有比之前预期更高的 81Kr 裂变产率。在地壳成分平均（即中等 U 浓度）的不太极端的环境中，