Implications of K, Cu and Zn isotopes for the formation of tektites

1,2,3Yun Jiang,3Heng Chen,3Bruce Fegley Jr.,3Katharina Lodders,4Weibiao Hsu,5Stein B.Jacobsen,3,5KunWang(王昆)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.003]
1CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China
2CAS Center for Excellence in Comparative Planetology, China
3Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
4Space Science Institute, Macau University of Science and Technology, Macau
5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
Copyright Elsevier

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.

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