Impact Induced Ballen Fracture Networks and their Formation in SiO2 Clasts

Anna Chanou, University of Western, of Earth Sciences, London, ON, Canada, Richard A F Grieve, Natural Resources Canada, Ottawa, ON, Canada; University of Western Ontario, Department of Earth Sciences, London, ON, Canada and Gordon R Osinski, University of Western Ontario, Physics and Astronomy, London, ON, Canada; University of Western Ontario, Earth Sciences, London, ON, Canada

Contact First Author: Anna Chanou; anna.chanou@gmail.com

Abstract ID#: 36627

 

English Abstract:
Rocks affected by meteoritic impacts display impact-related microtextures as the result of shock processes, as well as the associated extreme temperatures. So-called “ballen” is a form of microfracture network observed exclusively in SiO2 polymorphs. Ballen formation is expressed in the literature as a series of multiple steps and transformations between the low- and high polymorphs of quartz and mainly cristobalite; (Path 1) a solid-solid transition: a-quartz à diaplectic glass and then formation/crystallizationà β-cristobalite à α-cristobalite/ α-quartz; (Path 2) a solid-liquid transition: α-quartz à lechatelierite à β-cristobalite/β-quartz ballen à α-cristobalite/α-quartz. All variations of this model require multiple steps and transformations that would eliminate any pre-existing micro-structures, such as planar deformation features (PDF). PDF-bearing ballen were described and characterized under the optical microscope, as well as using electron microprobe and Raman spectroscopy. All ballen-bearing clasts were observed in impact melt-bearing breccias from the following impact structures Popigai, Siberia Russia; Mistastin and Deep Bay, Canada; and were always included in impact melt glass or vitric clasts. The PDF appear to be continuous through the cracks, as well as continuous between ballen-ridden parts of the clast and clear parts of the same clast. An additional formation mechanism of ballen fractures is proposed as the result of the above observations where ballen fractures form due to thermal shock without the need of the cristobalitization process and an amorphous phase. Our observations are in agreement with such a mechanism. In addition, we suggest that ballen development has no direct or indirect association to pressure levels and is only indicative of extreme thermal gradient. Finally, ballen although strongly suggestive of meteoritic impact it is not a diagnostic indicator and has been reported in other settings such as fulgurites.
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