Abstract
AbstractQuartz and cristobalite ballen aggregates surrounded by dendritic cristobalite in gneiss clasts of impact melt rocks from the Ries impact structure are analyzed by Raman spectroscopy, microscopy, and electron backscattered diffraction to elucidate the development of the characteristic polycrystalline ballen that are defined by curved interfaces between each other. We suggest that the investigated ballen aggregates represent former fluid inclusion‐rich quartz grains from the granitic gneiss protolith. Upon shock loading, they transformed into an amorphous phase that partly retained information on the precursor structure. Volatiles from inclusions dissolved into the amorphous phase. During decompression and cooling, dehydration takes place and causes fracturing of the amorphous phase and disintegration into small globular ballen, with the fluid being expelled along the fractures. A similar formation of small globules due to dehydration of silica‐rich glass is known for perlitic structures of volcanic rocks. Remnants of the precursor structure are present in the amorphous phase and enabled topotactic crystallization of quartz, leading to a crystallographic preferred orientation. Crystallization of more distorted parts of the amorphous phase led to random orientations of the quartz crystals. Ballen comprised of cristobalite formed from a dehydrated amorphous phase with no structural memory of the precursor. Dendritic cristobalite exclusively occurring at the rim of quartz ballen aggregate is interpreted to have crystallized directly from a melt enriched in fluids that were expelled during dehydration of the amorphous phase.
Highlights
Introduction“Ballen” 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 (T ≥ 1200 °C, e.g., Short 1970; Engelhardt 1972; Carstens 1975; Bischoff and Sto€ffler 1984; Rehfeldt-Oskierski et al 1986; French 1998; Osinski 2004; Ferriere et al 2008, 2009, 2010; Buchner et al 2010; Chen et al 2010; Schmieder et al 2014; Chanou et al 2015)
Based on polarized light microscopy, Bischoff and Sto€ffler (1984) distinguished three types of ballen, which they interpreted to crystallize at various shock pressures from an impactgenerated diaplectic glass that can preserve the shape of the original quartz grain and retains remnants of the original crystalline structure
Topotactic crystallization of the diaplectic glass with preserved short range order of the precursor structure leads to the dominating orientation of quartz, whereas crystallization from the more strongly disordered amorphous phase leads to crystallization of quartz with random orientation
Summary
“Ballen” 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 (T ≥ 1200 °C, e.g., Short 1970; Engelhardt 1972; Carstens 1975; Bischoff and Sto€ffler 1984; Rehfeldt-Oskierski et al 1986; French 1998; Osinski 2004; Ferriere et al 2008, 2009, 2010; Buchner et al 2010; Chen et al 2010; Schmieder et al 2014; Chanou et al 2015). Various models for the formation of the ballen aggregates have been proposed to date, which mostly include multiple silica phase transformations (Short 1970; Engelhardt 1972; Carstens 1975; French 1998; Ferriere et al 2009, 2010). The existing ballen classifications are based on the dominant phase comprising the ballen (quartz or cristobalite) and the optical extinction characteristics (Bischoff and Sto€ffler 1984; Ferriere et al 2008, 2009). Using EBSD, we will describe this microfabric, including the crystallographic orientations of quartz and cristobalite in ballen aggregates within granitic gneiss clasts from the Polsingen impact melt rocks of the Ries impact structure, Germany, and we discuss the conditions and the formation mechanisms of ballen based on these observations. Quartz ballen aggregates and dendritic rims of cristobalite around ballen aggregates are well known from the Ries (e.g., Engelhardt 1972; Ferriere et al 2009, 2010; Osinski 2004), cristobalite ballen aggregates have, to our knowledge, not been described from the Ries impact structure so far
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
