Chromite-plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites

摘要: Abstract Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL shows that unshocked chromite grains occur equant, subhedral rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted are fractured crushed during whole-rock brecciation. Others transected opaque veins; the veins form when impacts cause localized heating metal-troilite intergrowths above Fe-FeS eutectic (988°C), mobilization melts, penetration melt into fractures grains. Chromite-plagioclase assemblages nearly every shock-stage S3-S6 OC; range size from 20–300 μm consist 0.2–20-μm-size euhedral, subhedral, anhedral plagioclase glass composition. Plagioclase has low impedance to compression. Heat shock-melted caused adjacent melt; crystallized this melt. Those completely generally richer Al 2 O 3 than unmelted, matrix same meteorite. veinlets (typically 0.5–2 thick 10–300 long) typically vicinity chromite-plagioclase assemblages. The formed melts were injected neighboring grains; residual plagioclase-rich continued flow, eventually pooling pockets. Chromite-rich “chondrules” (consisting mainly olivine, plagioclase-normative mesostasis, 5–15 vol.% chromite) many shocked OC regolith breccias but they absent primitive type-3 OC. They may have impact melting chromite, mafic silicates higher-energy events. was jetted site droplets due surface tension. Crystallization these commenced flight, prior landing on parent-body surface. 25 out S1 petrologic type 5 6 I examined. Although rocks contain unstrained olivine sharp optical extinction, most possess other indicators such extensive darkening, numerous occurrences Cu, polycrystalline troilite, veins. It seems likely levels at least high S3 then annealed heat generated event. During annealing, crystal lattices healed survived. Published Ar-Ar age data for some indicate annealing events occurred very early history parent asteroids. common occurrence is consistent collisions being major mechanism responsible metamorphosing