Tektite and microtektite formation have important implications on our understanding of impacts both on Earth, the Moon and on other bodies within our solar system. Here, we investigate the formation mechanisms of microtektites by analysing the K isotope systematics and elemental compositions of forty-four Australasian microtektites from various distances from the proposed impact location. Based on the K isotope and concentration data, the microtektites analyzed here are split into two groups, the “ODP group” and the “MB group”. The ODP group were recovered from the Ocean Drilling Project (ODP) sediment cores and consist of microtektites which landed closer to the proposed impact site (∼1220–1240 km) and show limited δK variation (–1.06 ‰ to −0.21 ‰) and higher K concentrations (2.48 wt% to 3.66 wt% KO). In contrast, the MB group were mostly collected from the surface of Miller Butte (MB) in Antarctica and represent microtektites which landed significantly further from the proposed impact site (∼4100–10800 km) and contain large δK variations (−4.04 ‰ to 0.57 ‰) and low K concentrations (0.49 wt% to 1.45 wt% KO). For the microtektites studied here, the overall correlation observed is consistent with condensation whereby a greater extent of K depletion correlates with lighter K isotope compositions. This simple condensation model is in contrast to previous studies which find evidence for complex evolution involving evaporation, condensation, and mixing. For the ODP group microtektites, the isotopic and elemental data suggest condensation from an upper continental crust (UCC) starting composition. Conversely, for the MB group a UCC starting composition is not compatible, as even the most K-rich MB group microtektites are significantly depleted in K and display δK values much higher than the UCC. These observations can be explained by a vapor plume with a progressively evolving K isotope composition, with the earliest K condensates depleting and fractionating K within the plume, thus altering the starting K compositions for the later K condensates. From this data we calculate a cooling rate of up to 2,600 K/hour for the ODP group and up to 20,000 K/hour for the MB group, which are comparable to the cooling rates measured for tektites and considerably faster than those theoretically calculated or experimentally determined for chondrules. Overall, when assessed within the context of previous studies, microtektite formation appears very complex with evidence for different volatilization processes to different degrees observed within different microtektites. As such, while condensation appears dominant for K within the Australasian microtektites studied here, more work is needed to fully untangle the processes involved in microtektite formation.

中文翻译：

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了解微玻璃陨石的形成：蒸气羽流中凝结的钾同位素证据


玻璃陨石和微玻璃陨石的形成对于我们了解地球、月球和太阳系内其他天体的影响具有重要意义。在这里，我们通过分析距拟议撞击位置不同距离的 44 颗澳大利亚微玻璃陨石的 K 同位素系统学和元素组成来研究微玻璃陨石的形成机制。根据 K 同位素和浓度数据，此处分析的微玻璃陨石分为两组：“ODP 组”和“MB 组”。 ODP 组是从海洋钻探项目 (ODP) 沉积物岩心中回收的，由落在更接近拟议撞击地点（约 1220–1240 公里）的微玻璃陨石组成，显示出有限的 δK 变化（–1.06 ‰ 至 -0.21 ‰）和更高K 浓度（2.48 wt% 至 3.66 wt% KO）。相比之下，MB 组主要是从南极洲的米勒孤峰 (MB) 表面收集的，代表着距离拟定撞击地点 (∼4100–10800 km) 更远的微玻璃陨石，并且包含较大的 δK 变化（−4.04 ‰ 至 0.57） ‰）和低 K 浓度（0.49 wt% 至 1.45 wt% KO）。对于此处研究的微玻璃陨石，观察到的总体相关性与凝结一致，即较大程度的 K 消耗与较轻的 K 同位素组成相关。这个简单的冷凝模型与之前的研究形成鲜明对比，之前的研究发现了涉及蒸发、冷凝和混合的复杂演化的证据。对于 ODP 组微玻璃陨石，同位素和元素数据表明凝结物来自上大陆壳 (UCC) 起始成分。 相反，对于 MB 组，UCC 起始成分是不相容的，因为即使是最富含 K 的 MB 组微玻璃陨石也明显缺乏 K，并且显示出比 UCC 高得多的 δK 值。这些观察结果可以用具有逐渐演变的 K 同位素组成的蒸气羽流来解释，最早的 K 凝结物在羽流中耗尽并分馏 K，从而改变了后来的 K 凝结物的起始 K 成分。根据这些数据，我们计算出 ODP 组的冷却速率高达 2,600 K/小时，MB 组的冷却速率高达 20,000 K/小时，这与玻璃陨石测量的冷却速率相当，并且比理论计算或实验的冷却速率要快得多为球粒确定。总体而言，当在以前的研究背景下进行评估时，微玻璃陨石的形成显得非常复杂，有证据表明在不同的微玻璃陨石中观察到了不同程度的不同挥发过程。因此，虽然这里研究的澳大利亚微玻璃陨石中 K 的凝结似乎占主导地位，但还需要更多的工作来完全解开微玻璃陨石形成过程。