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Lithium in tektites and impact glasses: Implications for sources, histories and large impacts |
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| Authors: | T Magna A Deutsch R Skála J Mizera L Adolph |
| Institution: | a Institut für Mineralogie, Universität Münster, Corrensstr. 24, D-48149 Münster, Germany b Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1, Czech Republic c Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany d Institut für Geologie, Universität Bern, Baltzerstr. 1+3, CH-3012 Bern, Switzerland e Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, CZ-165 00 Prague 6, Czech Republic f Institut für Geowissenschaften, Universität Frankfurt, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany g Nuclear Physics Institute, Academy of Sciences of the Czech Republic, CZ-250 68 ?e?, Czech Republic |
| Abstract: | Lithium (Li) abundances and isotope compositions were determined in a representative suite of tektites (moldavites, Muong Nong-type tektites and an australite, Ivory Coast tektites and bediasites), impact-related glasses (Libyan Desert Glass, zhamanshinites and irghizites), a glass fragment embedded in the suevite from the Ries impact crater and sedimentary materials in order to test a possible susceptibility of Li to fractionation during hypervelocity impact events and to de-convolve links to their potential parental sources. The overall data show a large spread in Li abundance (4.7-58 ppm Li) and δ7Li values (−3.2‰ to 26.0‰) but individual groups of tektites and impact glasses have distinctive Li compositions.Most importantly, any significant high-temperature Li isotope fractionation can be excluded by comparing sedimentary lithologies from central Europe with moldavites. Instead, we suggest that Li isotope compositions in tektites and impact-related glasses are probably diagnostic of the precursor materials and their pre-impact geological histories. The Muong Nong-type tektites and australite specimen are identical in terms of Li concentrations and δ7Li and we tentatively endorse their common origin in a single impact event. Evidence for low-temperature Rayleigh fractionation, which must have operated prior to impact-induced melting and solidification, is provided for a subset of Muong Nong-type tektites. Although Li isotope variations in most tektites are broadly similar to those of the upper continental crust, Libyan Desert Glass carries high δ7Li ?24.7‰, which appears to mirror the previous fluvial history of parental material that was perhaps deposited in lacustrine environment or coastal seawater. Lithium isotopes in impact-related glasses from the Zhamanshin crater define a group distinct from all other samples and point to melting of chemically less evolved mafic lithologies, which is also consistent with their major and trace element patterns.Extreme shock pressures and the related extreme post-shock temperatures alone appear not to have any effect on the Li isotope systematics; therefore, useful information on parental lithologies and magmatic processes may be retrieved from analyses of Martian and lunar meteorites. Moreover, lack of significant Li depletion in tektites provides further constraints on the loss of moderately volatile elements during the Moon-forming impact. |
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