¹Masaaki Miyahara,^2,3Akira Yamaguchi,⁴Eiji Ohtani,⁵Naotaka Tomioka,⁶Yu Kodama
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13724]
¹Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima, 739-8526 Japan
²National Institute of Polar Research, Tokyo, 190-8518 Japan
³Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
⁴Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
⁵Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, Kochi, 783-8502 Japan
⁶Marine Works Japan, Nankoku, Kochi, 783-8502 Japan
Published by arrangement with John Wiley & Sons

High-pressure minerals in the eucrite Padvarninkai were investigated. Parts of anorthitic plagioclase and tridymite in the host rock of Padvarninkai vitrified, indicating that the shock pressure was 22–27 GPa. Tissintite, coesite, and a majorite-bearing garnet occurred in the shock-melt veins of Padvarninkai as high-pressure minerals. Tissintite, kyanite, corundum, and dense plagioclase have occurred in the anorthitic plagioclase grains. The anorthitic plagioclase was melted and tissintite crystallized from the melt after the crystallization of kyanite and corundum. The residual melt became dense plagioclase by quenching. Tridymite has also melted and coesite crystallized from the melt. The formation of tissintite and coesite indicates that the shock pressure recorded in the veins was 2–13 GPa. The temperature increased drastically in the veins (>˜3000 K) compared with the host rock (<˜800 K). Parts of the tissintite and coesite became, respectively, amorphous (or anorthite) and quartz. Two different impact events may be recorded in Padvarninkai: The first impact event brecciated a part of the host rock, and the second impact event induced the melting of the brecciated portion, resulting in the formation of shock-melt veins where the conditions are a high temperature and a relatively low pressure. In the veins, tissintite and coesite formed first, and parts of them underwent a back-transformation due to a long cooling time.

¹Tomáš Magna,^2,3Yun Jiang,⁴Roman Skála,²Kun Wang,⁵Paolo A.Sossi,⁴Karel Žák
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.07.022]
¹Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic
²Department 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
³CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing CN-210033, China
⁴Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, CZ-16500 Prague 6, Czech Republic
⁵Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
Copyright Elsevier

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 2,500K and <0.002‰ change in the ⁴¹K/³⁹K ratio even for a small sphere of 0.002 m at 2,500K, 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 (δ⁴¹K 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 (δ⁴¹K 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 δ⁴¹K 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 δ⁴¹K 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.