Implications of K, Cu and Zn isotopes for the formation of tektites
Implications of K, Cu and Zn isotopes for the formation of tektites
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K、Cu 和 Zn 同位素对玻璃陨石形成的影响
DOI：
10.1016/j.gca.2019.06.003
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发表时间：
2019
影响因子：
5
通讯作者：
Wang Kun
中科院分区：
文献类型：
--
作者：
Jiang Yun;Chen Heng;Fegley Jr Bruce;Lodders Katharina;Hsu Weibiao;Jacobsen Stein B.;Wang Kun
陨石是一种毫米至厘米大小的玻璃状物体，是由高能流星体在高温高压和还原条件下撞击地球表面而产生的。它们是地球历史上大规模灾难性事件的产物，可以用来理解中等挥发性元素（如K和Zn）在撞击汽化事件中的行为。在这里，我们报告了来自三个不同散布场的岩屑的总体K同位素组成，以及一个完整岩屑中K和Zn同位素的“原位”剖面分析。所有岩屑的K同位素组成范围较窄（δ41KBSE:−0.10 ± 0.03‰~ 0.16 ± 0.04‰），未显示大块硅酸盐土（BSE）和上陆壳物质的K同位素分馏，与前人研究结果一致。相比之下，即使在一个样品内，Zn同位素也表现出很大的变化（δ66Zn:−0.39 ± 0.02‰至2.38 ± 0.03‰）。为了对中等挥发性元素（K， Zn和Cu）的不同行为提供连贯的解释，我们进行了热化学计算，计算了溶解在硅酸盐熔体中的Cu2O， K2O和ZnO的分蒸汽压作为温度，压力，氧和氯逸度的函数。计算表明，在较大的参数空间范围内，Cu和Zn比K更容易汽化，从而产生较大的同位素分馏。相比之下，亲石元素K由于其在熔体中的活度系数非常低，更容易留在硅酸盐熔体中，因此K同位素保持未分馏状态。该研究为岩屑的形成提供了新的约束条件，并与解释岩屑中水分和挥发物极端耗竭的“气泡剥离”模型相一致。
Tektites are mm to cm sized glassy objects generated through high-energy meteoroid impacts on the surface of the Earth under high temperature and pressure, and reducing conditions. They are the products of large-scale catastrophic events in Earth’s history and can be used to understand the behavior of moderately volatile elements (e.g., K and Zn) during impact vaporization events. Here, we report bulk K isotopic compositions of tektites from three different strewn fields and “in-situ” profile analysis of both K and Zn isotopes in one complete tektite. All tektites span a narrow range in their K isotopic compositions (δ41KBSE: −0.10 ± 0.03‰ to 0.16 ± 0.04‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) and upper continental crust materials, which is consistent with previous results. In contrast, Zn isotopes show a large variation (δ66Zn: −0.39 ± 0.02‰ to 2.38 ± 0.03‰) even within one specimen. In order to provide a coherent explanation for the different behavior of moderately volatile elements (K, Zn and Cu), we have conducted thermochemical calculations to compute the partial vapor pressures of Cu2O, K2O, and ZnO dissolved in silicate melts as a function of temperature, pressure, oxygen and chlorine fugacities. In a large range of the parameter space, the calculations show that Cu and Zn can be vaporized much easier than K and thus produce large isotopic fractionation. In contrast, the lithophile element K is more prone to remain in the silicate melt because of its very low activity coefficient in the melt, and thus the K isotopes remain unfractionated. This study provides new constraints on the formation of tektites and is consistent with a “bubble-stripping” model to explain the extreme water and volatiles depletion in tektites.