Chemistry, small-scale inhomogeneity, and formation of moldavites as condensates from sands vaporized by the Ries impact

To elucidate the production of the primary melt units by the Ries impact, in the second section the geologic situation of the Ries area is considered with regard to possible source materials. The site of the Ries impact, situated at the northern border of the pre-Alpine Molasse basin, was covered at the time of the impact by fluviatile sediments of the lower sequence of the Obere Süßwasser-Molasse (OSM) Formation of Middle Miocene age, consisting of sands, marly sands and clays. Chemical analyses for major and trace elements of 38 moldavites from Bohemia and Moravia, and of 28 samples of the OSM Formation, collected from outcrops and drill holes, immediately south of the Ries crater and south of the Danube, show chemical conformity of moldavites and sand samples with regard to the major parameters SiO2,Al2O3+ FeO, and MgO + CaO, indicating that these sands were the essential source material of moldavites. But, differences in contents of individual major and trace elements between moldavites and sands show that the formation of moldavites from sands involved a specific chemical differentiation which can not be explained by selective vaporization or melting, nor by selective condensation from melt or vapeur. Because large ions were enriched and small ones depleted in moldavites, the ionic radius has controlled the distribution of elements between sands and moldavites. We assume that moldavites originated from a plasma that the impact produced at its encounter with the surficial sand formation at the impact site, and that the primary units formed as early condensates in which large ions were preferentially trapped.