Impact melt rocks from the Ries structure, Germany: an origin as impact melt flows?

Abstract

The production of impact melt rocks and glasses is a characteristic feature of hypervelocity impact events on Earth and other planetary bodies. This investigation represents the first detailed study of an unusual series of coherent impact melt rocks intermittently exposed around the periphery of the ~24-km diameter, ~14.5 Ma Ries impact structure, Germany. Optical and analytical scanning electron microscopy (SEM) reveals that the groundmass comprises sanidine, plagioclase, quartz and ilmenite (decreasing order of abundance) with the interstices filled by either fresh or devitrified glassy mesostasis. Primary crystallites display skeletal, dendritic and/or spherulitic textures indicating rapid crystallization from a melt. The mesostasis is characterized by extreme chemical heterogeneity (e.g., FeO and Al 2O 3 contents from ~1 to ~62–80 wt.%). This is likely due to a combination of crystal–liquid fractionation during rapid cooling and crystallization of an originally incompletely homogenized melt. Vapor phase crystallization of sanidine and cristobalite occurred in miarolitic cavities during late-stage cooling of the impact melts. The most likely protolith for the impact melt rocks are granitic rocks present in the crystalline basement target. The high volatile content of the mesostasis suggests that a large volatile component was retained from this protolith. Field observations together with analytical data and micro-textures indicate that the Ries impact melt rocks were molten at the time of, and after, deposition. Field relations with other impactites also suggest that these rocks were emplaced subsequent to the excavation stage of crater formation and that they are not, therefore, ballistic ejecta. Thus, it is proposed that the Ries impact melt rocks were emplaced as ground-hugging impact melt flows that emanated from different regions of the evolving transient cavity during the modification stage of crater formation. This is consistent with, and in fact predicted, by field relations, which indicate that these rocks overlie unequivocal ejecta. This represents the first recognition of impact melt flows near the rim of a terrestrial impact structure and is consistent with observations of such features on the Moon and Venus.