NEW EVIDENCE FOR IMPACT-INDUCED HYDROTHERMAL ACTIVITY IN THE MIOCENE RIES IMPACT CRATER, GERMANY

Post-impact hydrothermal activity has been reported from a number of terrestrial impact craters, from large impact structures 200-250 km in diameter to smaller craters only 1.8 km in diameter. Nonetheless, hydrothermal activity in the 24 km-sized Ries Impact Crater, one of the best investigated impact structures on Earth, has poorly been constrained. To date, mineralogical investigations point to a restriction of hydrothermal activity to the impact-melt bearing breccia, i.e. the crater-fill suevite. The formation of hydrous clay minerals in the fallout suevite, previously assigned to hydrothermal alteration, however, can alternatively be explained by late meteoric diagenesis. Here, we present new petrographic and geochemical data that indicate a post-impact hydrothermal activity in the Ries basin more extended than previously assumed. Specifically, carbonates of the Erbisberg, a spring mound located upon the Inner Crystalline Ring of the crater, shows streamer carbonates and cement crust stromatolites not seen in any of the previously investigated sublacustrine soda lake spring mounds of the Ries basin. Indeed, the fabric of the streamer carbonates, composed of subhorizontal slightly undulating tubes encrusted by dendroid arrays of calcite crystals, is almost identical to present-day streamer carbonates of hydrothermal springs. With respect to REE pattern, cement crust stromatolites show flat pattern, while streamer carbonates either were enriched in HREE due to diagenetic dissolution and reprecipitation, or exhibited flat REE pattern with a minor positive Eu anomaly. A post-impact calcite vein from brecciated gneiss of the subsurface crater floor finally revealed a flat REE pattern with a clear positive Eu-anomaly, considered to indicate hydrothermal remobilisation of Eu²⁺ from plagioclase. The chemostratigraphic position of the travertine mound, as indicated by ⁸⁷Sr/⁸⁶Sr values of mound margin carbonates with lacustrine components, suggests that the mound was active during deposition of the laminite unit of the basinal succession, i.e., at least several thousands of years after the impact event. A 65-m-deep research drilling in May 2011 by the Bavarian Geological Survey now provides the possibility to investigate hydrothermal and microbial signatures from the crystalline basement throughout the travertine mound.