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Potassium elemental and isotope constraints on the formation of tektites and element loss during impacts
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SYSNO ASEP 0547317 Document Type J - Journal Article R&D Document Type Journal Article Subsidiary J Článek ve WOS Title Potassium elemental and isotope constraints on the formation of tektites and element loss during impacts Author(s) Magna, T. (CZ)
Jiang, Y. (CN)
Skála, Roman (GLU-S) RID, SAI, ORCID
Wang, K. (CN)
Sossi, P. A. (CH)
Žák, Karel (GLU-S) SAI, RID, ORCIDSource Title Geochimica et Cosmochimica Acta. - : Elsevier - ISSN 0016-7037
Roč. 312, November (2021), s. 342-321Number of pages 22 s. Publication form Print - P Language eng - English Country GB - United Kingdom Keywords Potassium isotopes ; Tektite ; Moldavite ; Sediment ; Ries crater ; Evaporation OECD category Geology R&D Projects GA17-27099S GA ČR - Czech Science Foundation (CSF) GA13-22351S GA ČR - Czech Science Foundation (CSF) Method of publishing Limited access Institutional support GLU-S - RVO:67985831 UT WOS 000704172000004 EID SCOPUS 85112502575 DOI 10.1016/j.gca.2021.07.022 Annotation Potassium elemental and isotope systematics were investigated for a suite of central European tektites from three strewn sub-fields in Czech Republic and possible parent sedimentary materials from the vicinity of the Ries impact structure in SE Germany, supplemented by data for several other impact-related materials (bediasites, Ivory Coast tektites, Libyan Desert Glass). This is paralleled by computation of potential K loss and attendant isotope fractionation for physico–chemical conditions typical for formation of tektite precursor melts. These theoretical calculations indicate a <0.1% loss of K from tektite precursor melts up to 2,500 K and <0.002‰ change in the 41K/39K ratio even for a small sphere of 0.002 m at 2,500 K, precluding any significant K loss and isotope fractionation. Numerical modelling also indicates that differential velocities between surrounding gas and liquid are not sufficient to remove the gaseous boundary layer, such that the partial pressure of potassium developed around the molten moldavite beads impedes further evaporation and also contributes to back-condensation of the already evaporated potassium. Central European tektites (moldavites) are enriched in K compared to the assumed sedimentary sources from the wider Ries area whereby the latter materials do not exceed 2.9 wt.% K2O compared to 2.5–4.1 wt.% K2O in moldavites. The apparent K enrichment in moldavites may be explained by a yet unaccounted process during formation of tektite precursor melts and/or unidentified source, such as volcanoclastic deposits that were produced by large Mid-Miocene volcanic centers in the Pannonian Basin. The K isotope compositions of tektites are more variable than those of sediments from the wider Ries area but they largely overlap (d41K from –0.78 ± 0.03‰ to –0.13 ± 0.03‰ versus –0.72 ± 0.03‰ to –0.28 ± 0.02‰, respectively). These ranges mimic 41K/39K variations reported for igneous and sedimentary portions of the upper continental crust (d41K roughly between –0.7 and –0.1‰). They show a slight difference among the three investigated strewn sub-fields, depending on their respective distance from the impact. In detail, moldavites from the closest strewn sub-field in the Cheb Basin show predominantly heavy K isotope compositions and those from the farthest strewn sub-field in Western Moravia are uniformly isotopically light. The origin of this difference may reflect lithological heterogeneity of the target area. Potassium contents in bediasites and Ivory Coast tektites range between 1.3 and 1.8 wt.% K2O and their corresponding d41K values vary from –0.57 ± 0.02‰ to –0.41 ± 0.03‰. Both ranges are significantly narrower than those observed for moldavites. When compared to data for possible sedimentary precursors in the Chesapeake Bay and Bosumtwi impact structure, respectively, it is apparent that these tektites were neither depleted nor enriched in potassium. The extent of their K isotope fractionation relative to plausible sources remains unconstrained. The Libyan Desert Glass displays invariant d41K of ~ –0.57 ± 0.06‰ at ≤0.01 wt.% K2O. Given the silica-rich nature of LDG and the lack of possible parent materials, no further constraints can be placed at present to further resolve the source material or reveal details of LDG formation process. Workplace Institute of Geology Contact Jana Popelková, popelkova@gli.cas.cz, Sabina Janíčková, Tel.: 233 087 272 Year of Publishing 2022 Electronic address https://www.sciencedirect.com/science/article/pii/S0016703721004403
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