Impact Melt Ro Cks from the Ries Impact Structure, Germany: Preliminary Results of an Analytical Sem Study

Introduction: Impact melted materials occur in two main forms in the Ries impact structure [1]: (1) as isolated bodies of reddish, vesicular, fine-grained impact melt rock; (2) as in dividual glass y particles or c lasts in melt-bearing breccias or suevites. The latter lithologies have been the subject of a considerable number of investigations over the past few decades. In contrast, no detailed investigations of the impact melt rocks have been published. Here, we present the preliminary results of an analytical scanning electron microscope (SEM) study of impact melt rocks from the Ries structure. The Ries impact structure: The 24 km diameter 14.5 Ma old Ries impact structure is one of the best preserved complex impact structures on Earth. The target rocks consist of a flat-lying sequen ce of pred ominantly Meso zoic sedimentary rocks that unconform ably overlie Hercynian crystalline basement. The sedimentary cover at the time of impact varied in thickness from ~47 0 m in the north to ~820 m in the south [2]. The structure possesses a sequence of impactites (impact-produced rocks), including a thick series of crater-fill rocks, various types of dista l ejecta depos its (preserved up to ~37 km radius), and a tektite (‘molda vite ‘) strewn field extending out to distanc es of 260-400 km to the east of the Ries structu re [3]. Impact melt rocks: Only two occurrences of impact melt rock sensu stricto have been documented at the Ries impact structure. These occur at Amerbach and Polsingen in the ENE of the structure and represent isolated bodies of impact melt rock with lateral extents <10-50 m [1]. They are seen to overlie Bunte breccia or megablocks [1]. Previous workers have described these lithologies as consisting of a reddish , vesicular, fine-g rained recrystallized matrix of feldspar, celadonite, pyroxene and cristobalite [1, 4]. These rocks have also been termed (red) suevites by some workers [4, 5]. Samples and analytical techn iques: Four samples from the Polsingen quarry were collected by GRO during two field expeditions (August 2000 and June 2001 ). The Amerbach road cuts are no longer exposed, however, three samples from the impact collection of the Geo logical Survey of C anada we re examine d. Polished thin sections were investigated using a JEOL 6400 digital scanning electron microscope (SEM) equipped with a Link Analytical eXL energy dispersive spectrometer (EDS) and Si(Li) LZ-4 Pentafet detector. Phase com positions were determined in EDS mode at count times of 100s, beam operating conditions of 15 kV and 2.5 nA, and at a working distance of 37 mm. SEM back-scattered electron (BSE) imaging was used to investigate the microtextures.