Multistage compound chondrules and molded chondrules in the Bovedy (L3) meteorite

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Chondrites, Chondrule, Metamorphism (Geology), Petrography, Temperature Effects, Melts (Crystal Growth), Olivine, Shock Heating, Textures

Scientific paper

Petrographic examination of the Bovedy (L3) chondrite has revealed two interesting types of textural features that have not received much recent attention. They are compound chondrules with several superimposed secondary caps, and deformed chondrules that appear to be molded against each other in a manner resembling pillow basalt. We describe them both and comment on their significance. Two multistage compound chondrules were found. One is a 1-mm fragment of a barred olivine chondrule showing three successive caps on a primary. Each cap is finer than the one beneath it, and the olivine bars and blebs are optically continuous throughout. The other (1.5 mm) also has four stages but is composed of radiating pyroxene dendrites with additional skeletal olivine bars in the first two stages. The second stage completely encloses the partly disrupted first stage, while the third and fourth are bulbous, superimposed caps. The whole body is rimmed by metal. The question of whether chondrules were hot and plastic or cold and brittle at the time of accretion has important implications for the thermal history of chondrites. Cold accretion requires later reheating to induce equilibration; hot accretion leads to annealing during slow cooling. Evidence cited for hot accretion is the deformed, molded shape of some droplet chondrules. Although Bovedy is shocked, textural features survive, which are difficult to explain other than by hot accretion. Critical evidence we found is the presence of whole-droplet chondrules (not fragments of chondrules or compound chondrules) with indented or cuspate borders and a coherent pattern of crystallites that must therefore have grown after the chondrule shape was imposed.

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