Abstract Despite their economic and scientific significance, the tectonic drivers that lead to the genesis of kimberlites and related rocks remain enigmatic. The presence of any spatio-temporal relationship(s) between the rift, drift and collision history of continents and the emplacement of kimberlites can only be elucidated with accurate geochronological constraints. In this study, we evaluated the three most common radiogenic dating methods used to determine the emplacement ages of kimberlites and related rocks - Rb Sr phlogopite, U/Pb perovskite and 40Ar/39Ar phlogopite. We selected minimally altered samples from the Kaavi-Kuopio kimberlites, ultramafic lamprophyres and kimberlites from Kuusamo, and olivine lamproites from the Lentiira-Kuhmo-Kostomuksha cluster, all located in Finland. Our 33 new age determinations indicate that the olivine lamproites and ultramafic lamprophyres were emplaced on the Karelian craton during the interval between ~1180–1210 Ma, the Kuusamo kimberlites between ~730–750 Ma, and the Kaavi-Kuopio kimberlites between ~585–620 Ma. Our results show that the Rb Sr method regularly yields reproducible eruption ages from either magmatic or macrocrystic (xenocrystic) mica. In addition, the method can be employed on a single population of mineral separates using a sequential acid-leaching technique, without the need for an estimate of 87Sr/86Sr(i). The U Pb perovskite technique typically requires significant correction for ‘common-Pb’ i.e. Pb incorporated at the time of mineral formation and unrelated to subsequent radiogenic ingrowth. Here, we utilised isotope dilution analyses of co-existing spinel to extend the ‘spread’ in U Pb isochron space obtained by in situ LA-ICP-MS analyses of perovskite and enhance the accuracy and precision of intrusion ages. Our tests also show that two-point U Pb isochrons based on solution analyses of perovskite and spinel generally provide anomalously older ages. Relative to Rb Sr and U Pb results from the same kimberlite, we show that apparently precise 40Ar/39Ar plateau-ages, determined from step-heating experiments, are often anomalously ‘old’, which is attributable to 39Ar recoil effects. In cases where the 40Ar/39Ar step-heating spectra are even marginally ‘disturbed’, 40Ar/39Ar total-gas ages compare favourably with ages from other geochronometers and appear to provide accurate estimates for the emplacement age of kimberlites and related rocks. Overall, there is good agreement between the three geochronometers using the above approaches, which indicates that Rb Sr phlogopite, U Pb perovskite and 40Ar/39Ar phlogopite dating can provide robust age constraints for the emplacement of kimberlites and related rocks. The geochronometer of choice will rely on the freshness, abundance and size of datable material. Wherever possible, the application of multiple geochronometers is recommended.

中文翻译：

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金伯利岩和相关岩石常用地质年代学方法的比较：来自芬兰的三个案例研究

摘要 尽管具有经济和科学意义，导致金伯利岩和相关岩石成因的构造驱动因素仍然是个谜。大陆的裂谷、漂移和碰撞历史与金伯利岩就位之间的任何时空关系的存在只能通过准确的地质年代学约束来阐明。在这项研究中，我们评估了用于确定金伯利岩和相关岩石的侵位年龄的三种最常见的放射成因测年方法 - Rb Sr 金云母、U/Pb 钙钛矿和 40Ar/39Ar 金云母。我们从 Kaavi-Kuopio 金伯利岩、Kuusamo 的超镁铁质菱镁矿和金伯利岩，以及 Lentiira-Kuhmo-Kostomuksha 群的橄榄石菱镁矿中选择了最小改变的样品，这些样品都位于芬兰。我们对 33 个新年龄的测定表明，橄榄石菱镁矿和超镁铁质菱镁矿在约 1180-1210 Ma 之间的时间间隔内、约 730-750 Ma 之间的 Kuusamo 金伯利岩和约 585-620 Ma 之间的 Kaavi-Kuopio 金伯利岩被安置在卡累利阿克拉通上嘛。我们的结果表明，Rb Sr 方法定期从岩浆或巨晶（异晶）云母中产生可重复的喷发年龄。此外，该方法可用于使用顺序酸浸技术的单一矿物分离物群，而无需估计 87Sr/86Sr(i)。U Pb 钙钛矿技术通常需要对“普通铅”进行显着校正，即在矿物形成时掺入的铅，与随后的放射性向内生长无关。这里，我们利用共存尖晶石的同位素稀释分析来扩展由钙钛矿原位 LA-ICP-MS 分析获得的 U Pb 等时线空间中的“扩散”，并提高侵入年龄的准确性和精度。我们的测试还表明，基于钙钛矿和尖晶石溶液分析的两点 U Pb 等时线通常提供异常古老的年龄。相对于来自同一金伯利岩的 Rb Sr 和 U Pb 结果，我们表明，由阶梯加热实验确定的明显精确的 40Ar/39Ar 平台年龄通常异常“古老”，这可归因于 39Ar 反冲效应。在 40Ar/39Ar 阶梯加热谱甚至轻微“扰动”的情况下，40Ar/39Ar 总气体年龄与其他地质年代计的年龄相比具有优势，并且似乎为金伯利岩和相关岩石的侵位年龄提供了准确估计。总体而言，使用上述方法的三个地质年代计之间有很好的一致性，这表明 Rb Sr 金云母、U Pb 钙钛矿和 40Ar/39Ar 金云母定年可以为金伯利岩和相关岩石的就位提供强有力的年龄约束。选择的地球计时仪将取决于可数据材料的新鲜度、丰度和大小。在可能的情况下，建议使用多个地球计时仪。