Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.548w&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 548-549
Other
2
Allende Meteorite, Chondrule, Cooling, Meteoritic Microstructures, Pyroxenes, Crystallization, Lamella (Metallurgy), Olivine, Phase Diagrams, Temperature Effects, Textures, Transmission Electron Microscopy
Scientific paper
Cooling rates of chondrules have been derived from dynamic crystallization experiments. In these experiments a cooling rate between 100 and 2000 C/hr was inferred for granular and porphyritic olivine chondrules, if both chondrule texture and chemical zoning of olivine is taken into account. We present a new approach for the determination of cooling rates of granular olivine pyroxene (GOP) chondrules based on the microstructure of chondrule minerals. All observations were made on Allende chondrules. The lamellar intergrowth of pigeonite (Pig) and diopside (Di) on (001) is frequently observed in Allende GOP chondrules, and an average wavelength between 25 and 30 nm is obtained. A time-temperature-transformation (TTT) diagram for the coarsening of Pig/Di lamellae is presented. It should be mentioned that the results were obtained from isothermal experiments, and thus, cannot be directly applied to systems undergoing exsolution during continuous cooling. These data can be used to infer the order of magnitude of cooling rates. In the temperature interval of 1350-1100 C, cooling rates between approximately 10 and 1 C/hr are appropriate for the observed wavelengths of 25-30 nm. A second constraint on the cooling rate is obtained from size of b-Antiphase Domains (APDs), which are frequently observed in Allende chondrule plagioclase. According to experimental data, the observed domain sizes (10-30 nm) would limit cooling rates to approximately 10-0.5 C/hr. This cooling rate is based on the assumption that plagioclase crystallized at a temperature of 1350 C. Cooling rates of GOP chondrules in Allende estimated from the wavelength of Pig/Di exsolution lamellae and the size of b-APDs in plagioclase are on the order 10-1 C/hr in the temperature range of 1350-1100 C, whereas estimates derived from dynamic crystallization experiments are between 100 and 2000 C/hr.
Mueller Wolfgang F.
Weinbruch Stephan
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