Abstract The Acasta Gneiss Complex (AGC) of the Northwest Territories, Canada, contains some of the oldest evolved terrestrial crust and has a sustained record of more than a billion years of magmatism (>4.0–2.9 Ga). These rocks provide an opportunity to investigate the nature of continental crust formation on the early Earth. Because complexities in zircons and bulk rocks are characteristic of the early Earth record—including the AGC—strategies are needed to extract accurate and meaningful age and isotopic information. In order to evaluate the accuracy of the early Earth Hf isotope record, we examined AGC zircons from a range of lithologies using paired chemical abrasion isotope dilution U-Pb age and solution Lu-Hf isotope analysis and compared these with previous results obtained using laser ablation split-stream (LASS) analysis. We describe an approach whereby LASS is used to identify rocks with the least complex zircons, and, when appropriate, solution methods are then used to refine the age and Hf isotopic composition to the highest precision. This two-pronged analytical approach results in a more robust determination of the age and Hf isotopic record of complex rocks and zircons and allows identification of complexity in the Hf isotopic record that would not be apparent by solution analysis alone, thereby refining the record of magmatic evolution on the early Earth. Despite the better precision, solution techniques are unsuitable for rocks with complexly zoned zircons. In particular, zircons from the two oldest AGC rocks in this study have positive initial eHf values when determined by solution analysis (+2.8 and +4.9 at 4.0 and 3.9 Ga, respectively) but have negative eHf values when determined by LASS (−3.4 and −4.7, respectively). This is attributable to the presence of later radiogenic overgrowths on the zircon grains which are incorporated in solution analyses but can be avoided using LASS. This provides important clarity to the AGC Hf isotope record. In total, all but one of the AGC rocks we analyzed, including the oldest samples, have negative eHf values and indicate derivation from an enriched reservoir; none of these samples—in contrast to whole rock Nd isotope compositions—have sufficiently positive eHf values to indicate their derivation from a depleted mantle reservoir. Despite the presence of ancient AGC crust, there is no record of corresponding mantle depletion. This implies that extraction of Hadean crust in this region did not happen in sufficient volume to result in widespread mantle depletion in the AGC source by the Eoarchean. Our results underscore the importance of identifying different components in ancient zircons—and the rocks that contain them—and accurately determining the age and isotopic composition of those components. This is critically important for clarifying the record of the formation of enriched crust and development of the depleted mantle in Earth’s early history.

中文翻译：

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从 4.0-2.9 Ga Acasta 片麻岩复合体中解开锆石的复杂性

摘要 加拿大西北地区的阿卡斯塔片麻岩杂岩 (AGC) 包含一些最古老的演化陆地地壳，并具有超过 10 亿年的岩浆活动记录 (>4.0-2.9 Ga)。这些岩石为研究早期地球大陆地壳形成的性质提供了机会。由于锆石和大块岩石的复杂性是早期地球记录（包括 AGC）的特征，因此需要采取策略来提取准确且有意义的年龄和同位素信息。为了评估早期地球 Hf 同位素记录的准确性，我们使用成对化学磨损同位素稀释 U-Pb 年龄和溶液 Lu-Hf 同位素分析检查了一系列岩性中的 AGC 锆石，并将这些与以前使用激光烧蚀获得的结果进行了比较分流 (LASS) 分析。我们描述了一种方法，其中 LASS 用于识别具有最不复杂锆石的岩石，然后在适当的时候使用求解方法将年龄和 Hf 同位素组成细化到最高精度。这种双管齐下的分析方法可以更可靠地确定复杂岩石和锆石的年龄和 Hf 同位素记录，并可以识别单独通过溶液分析无法明显看出的 Hf 同位素记录中的复杂性，从而完善岩浆的记录。早期地球的进化。尽管精度更高，但求解技术不适合具有复杂分区的锆石的岩石。特别是，本研究中两块最古老的 AGC 岩石中的锆石在通过溶液分析确定时具有正的初始 eHf 值（+2.8 和 +4.9 在 4.0 和 3.9 Ga，分别）但当由 LASS 确定时具有负的 eHf 值（分别为 -3.4 和 -4.7）。这归因于锆石颗粒上后期放射性过度生长的存在，这些过度生长被纳入溶液分析中，但可以使用 LASS 来避免。这为 AGC Hf 同位素记录提供了重要的清晰度。总的来说，我们分析的所有 AGC 岩石（包括最古老的样本）都具有负的 eHf 值，表明来自富集储层；与全岩 Nd 同位素组成相比，这些样品中没有一个具有足够正的 eHf 值来表明它们来自枯竭的地幔储层。尽管存在古老的AGC地壳，但没有相应地幔消耗的记录。这意味着该地区的冥古宙地壳的提取量不足以导致始太古代在AGC源中广泛地消耗地幔。我们的结果强调了识别古代锆石中不同成分以及包含它们的岩石并准确确定这些成分的年龄和同位素组成的重要性。这对于澄清地球早期历史中富集地壳的形成和贫化地幔的发展记录至关重要。