The fate of moderately volatile elements in impact events - Lithium connection between the Ries sediments and central European tektites
Abstract
Lithium abundances and isotope compositions are presented for a suite of sediments from the surroundings of the Ries Impact structure, paralleled by new Li data for central European tektites (moldavites) from several substrewn fields (South Bohemia, Moravia, Cheb Basin, Lusatia), including a specimen from the newly discovered substrewn field in Poland. The data set was supplemented by three clay fractions isolated from sedimentary samples.
Moldavites measured in this study show a very narrow range in δ7Li values (-0.6 to 0.3%o relative to L-SVEC) and Li contents (23.9-48.1 ppm). This contrasts with sediments from the Ries area which show remarkable range in Li isotope compositions (from - 6.9 to 13.4%o) and Li contents (0.6-256 ppm).
The OSM sediments which, based on chemical similarity, formed the major part of moldavites, show a range in δ7Li values from -2.0 to 7.9%o and Li contents from 5.8 to 78.9 ppm. Therefore, the formation of moldavites was apparently accompanied by large-scale mixing, paralleled by chemical and isotope homogenization of their parent matter.
The proposed Li mixing model indicates that sands, clayey sediments, and low volumes of carbonates are the major components for tektite formation whereas residual paleokarst sediments could have been a minor but important component for a subset of moldavites. Striking homogenization of Li in tektites, combined with limited Li loss during impacts, may suggest that moderately volatile elements are not scavenged and isotopically fractionated during large-scale collisions, which is consistent with recent models.
In general, whether homogenization of bodies with distinct Li isotope systematics takes place, or collision of bodies with similar Li systematics operates cannot be resolved at present stage but Li isotope homogeneity of solar system planets and asteroidal bodies tentatively implies the latter.