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- Title
SPHERULES AND MICROCRYSTITES IN ROCHECHOUART IMPACTOCLASTITE DIKES.
- Authors
Wittmann, Axel; Lambert, Philippe
- Abstract
Introduction: Impactoclastite, an ash-like material mainly composed of mineral debris with macroscopic layering, occurs exclusively in a ~2 km2 region ca. 4 km NW of the presumed center of the Rochechouart impact structure near the community of Chassenon [1]. A horizontally layered impactoclastite deposit devoid of impact melt with a particle size of 10‒150 μm occurred in a quarry capping suevite [1]. In contrast, variably oriented, mm to dm-thick impactoclastite dikes that contain impact melt particles cut the Chassenon suevite to a depth >30 m [2]. Notably, both impactoclastite lithologies contain impactor components [3]. We report additional observations to a study of Chassenon impactoclastite dike-hosted spherules [4] to unravel the petrogenesis of these impact vapor plume-related deposits. Samples and Methods: We analyzed three additional petrographic thin sections of three samples of a sub-vertical impactoclastite dike from Chassenon drillcore SC1 [2] with an optical microscope and a JEOL-JXA 8530F fieldemission electron microprobe equipped with an energy-dispersive spectrometer (EDS) at Arizona State University. Results: The sample at 2.52 m depth contains one 110 μm microcrystite (Fig. 1) with an amorphous core surrounded by 20 μm skeletal pyroxene laths and a 3 μm rim that contains sub-μm crystals enriched in Fe, Cr and Ni. We did not find spherules in our sample thin section at 3.09 m depth. The sample at 3.46 m depth contains five spherules; three are oval, 50 to 240 μm, and one is 400 × 120 μm and has a tear-drop shape; one of the oval spherules contains clusters of up to 2 μm MgAlFeCrNi spinel crystal near its rim; also, one dumbbell-microcrystite is present with sub-μm clusters of MgAlFeCrNi spinel near its rim. All spherules and microcrystites are pervasively altered and hydrated. Discussion: Spherules and microcrystites, some associated with MgAlFeNiCr spinel, are evidence for impact fireball components [5] in the Chassenon impactoclastite dikes [1], similar to deposits overlying suevite in the Bosumtwi and El'gygytgyn craters [6,7]. Contrary to these fireball deposits, spherules appear absent in the horizontally layered impactoclastite deposit that caps the Chassenon suevite; instead, sub-vertical impactoclastite dikes host spherules. Impactoclastite dike formation is constrained by their wallrock-parallel layering [1] that suggests rapid and forceful emplacement, likely precluding settling through water, while fallback of ~100 μm particles from an impact plume should occur within 1 day after impact [8]. Hypothesis: The Chassenon suevite/impactoclastite deposit was likely situated in a depression that allowed its preservation and present-day topographic prominence. Melt-free surficial impactoclastite could have settled early from the impact plume because it had separated from higher energetic, melt-rich ejecta; later fallback material in the horizontal impactoclastite was likely lost to erosion. Subvertical fissures in the suevite may have opened due to gravitational adjustment following central peak collapse. These block movements may have caused a mass deficiency in the crater floor during the fallback phase hours to a few days after the impact, ramping down the ~2 km² region and triggering sudden decompression in the crater floor [9], which sucked spherulebearing fallback material into these fissures. Acknowledgements: CIRIR for supplying samples of Rochechouart impact rocks.
- Subjects
ARIZONA State University; ELECTRON probe microanalysis; LUNAR craters; SPINEL group; DIKES (Geology); OPTICAL microscopes; COMMUNITIES; EROSION; SPINEL
- Publication
Meteoritics & Planetary Science, 2022, Vol 57, p6508
- ISSN
1086-9379
- Publication type
Article