“Ballen structures” of quartz and cristobalite aggregates have been observed in impactites from a number of terrestrial impact structures, predominantly from impact melt rocks, suevites, and target rock clasts affected by high post-shock temperatures (e.g., CARSTENS, 1975; FERRIÈRE et al., 2010). The aggregates range from few hundreds of microns to 5mm in size and have a peculiar fracture pattern. For their formation, phase transformations from cristobalite to quartz after shock have commonly been proposed (e.g., CARSTENS, 1975; FERRIÈRE et al., 2010). Raman-spectroscopy, light and electron microscopy, as well as electron back scattered diffraction (EBSD) analysis of quartz and cristobalite ballen aggregates in impact breccias from the Ries crater, however, suggest that they form by dehydration of diaplectic silica glass and a “diaplectic silica melt”, respectively. The polycrystalline quartz ballen aggregates partly preserve the crystallographic orientation and shape of the original quartz grain from the target granitic gneisses. Cristobalite with ballen structure is comprised by radiating polycrystalline aggregates. We suggest that the original quartz rich in fluid inclusions transformed into diaplectic glass or highly viscous supercooled “diaplectic melt” as a result of shock loading, where the volatiles from the inclusions dissolved into the glass or melt, and the glass retained remnants of the quartz structure guiding the subsequent topotactic crystallization.
During decompression and cooling, dehydration causes strain concentration followed by fracturing into small globular ballen. The fluid is expelled along the fractures, comparable to perlitic structures in silica rich volcanic rocks, which can be described as circular crack networks (e.g., MARAKUSHEV et al., 1987). Whereas quartz with ballen structure crystallized from the dehydrated diaplectic glass, cristobalite with ballen structure is suggested to have crystallized rather from the “diaplectic melt” with no crystallographic or shape memory, probably generated due to a locally higher fluid availability. Dendritic and radiating cristobalite at the rim of quartz ballen aggregates in contact to vesicles is suggested to have crystallized from a melt enriched in the expelled fluids from the diaplectic silica glass/melt. Prismatic cristobalite in contact to feldspar in the matrix surrounding the ballen aggregates is interpreted to have crystallized from a silica-rich melt.
Fabian Dellefant (1), Claudia A. Trepmann (1), Melanie Kaliwoda (2), Kai-Uwe Hess (1), Wolfgang W. Schmahl (1,2) & Stefan Hölzl (3)
Department of Earth and Environmental Sciences, LMU, München, Germany (1); Mineralogische Staatssammlung, SNSB, München, Germany (2); RiesKraterMuseum Nördlingen, Eugene-Shoemaker-Platz 1, D-86720, Nördlingen, Germany (3)