Carbonate-silicate liquid immiscibility upon impact melting: Ries Crater, Germany

The 24 km diameter Ries impact crater in southern Germany is one of the mos t studied impact structures on Earth. The Ries impactor struck a Triassic t o Upper Jurassic sedimentary sequence overlying Hercynian crystalline basem ent. At the time of impact (14.87 +/- 0.36 Ma; Storzer et al., 1995), the 3 50 m thick Maim limestone was present only to the south and east of the imp act site. To the north and west, the Maim had been eroded away, exposing th e underlying Dogger and Lias. The largest proportion of shocked target mate rial is in the impact-melt-bearing breccia suevite. The suevite had been be lieved to be derived entirely from the crystalline basement. Calcite in the suevite has been interpreted as a postimpact hydrothermal deposit. From optical inspection of 540 thin sections of suevite from 32 sites, I fi nd that calcite in the suevite shows textural evidence of liquid immiscibil ity with the silicate impact melt. Textural evidence of liquid immiscibilit y between silicate and carbonate melt in the Ries suevite includes carbonat e globules within silicate glass, silicate globules embedded in carbonate, deformable and coalescing carbonate spheres within silicate glass, sharp me nisci or cusps and budding between silicate and carbonate melt, fluidal tex tures and gas vesicles in carbonate schlieren, a quench crystallization seq uence of the carbonate, spinifex textured quenched carbonate, separate carb onate spherules in the suevite mineral-fragment matrix, and inclusions of m ineral fragments suspended in carbonate blebs. Given this evidence of liqui d immiscibility, the carbonate in the suevite therefore has-like the silica te melt-a primary origin by impact-shock melting. Evidence of carbonate-silicate liquid immiscibility is abundant in the suev ites from the southwest to east of the Ries crater. The rarer suevites to t he west to northeast of the crater are nearly devoid of carbonate melts. Th is correspondence between the occurrence of outcropping limestones at the t arget surface and the formation of carbonate melt indicates that the Maim l imestones are the source rocks of the carbonate impact melt. This correspon dence shows that the suevites preserve a compositional memory of their sour ce rocks. From the regional distribution of suevites with or without immisc ible carbonate melts, it is inferred that the Ries impactor hit the steep A lbtrauf escarpment at its toe, in an oblique impact from the north.