Are some chondrule rims formed by impact processes? Observations and experiments.

摘要: Abstract Observations and experimental evidence are presented to support the hypothesis that high-speed impact into a parent body regolith can best explain certain textures compositions observed for rims on some chondrules. A study of 19 interclastic rimmed chondrules in Weston ( H 3 4 ) ordinary chondrite shows two main rim types present porphyritic olivine-pyroxene (POP) pyroxene (PP) chondrules: granular opaque rims. Granular composed welded, fine-grained host chondrule fragments. Bulk vary among chondrules, but each is compositionally dependent chondrule. Opaque contain mineral glass distinctly different from those host, partially reacted mantle components, matrix grains. greatly enriched FeO (up 63 wt%). The original compositional zonation patterns euhedral grain outlines discontinuous at chondrule/rim interface. dominated by fayalitic olivine (Fa92-56), with high Al2O3 content (0.78–3.15%), which makes them primary olivine, similar Fe-olivine other meteorites. Thin zones minerals adjacent intermediate between Mg-rich interior Fe-rich rim, indicates reaction relationship. Regardless conclusions drawn regarding rims, characteristics appear be inconsistent nebular condensation, fragments included within rim. We have initiated series experiments, using Ames two-stage light gas gun, investigate result thermal mechanical alteration upon low-density medium. Clusters ∼ 200-μm-sized silicate particles were fired aerogel (density = 0.1 g cm−3) velocities 5.6, 4.7, 2.2 km sec−1. Recovered grains show range fragmented projectile mixed melted nearly clumps projectile. These results demonstrate rim-like projectiles high-velocity encounter target. Therefore, experiments appropriately chosen target materials provide test common possibly chondrites formed such process.