Dynamic crystallization study of barred olivine chondrules
Abstract
Chondrules, which comprise the bulk of most chondrites, formed very early in the solar system, and understanding their formation will elucidate early nebular processes. Barred olivine (BO) chondrules make up approximately 10% of chondrules and because of their unique dendritic textures represent a restricted set of crystal growth conditions and, thus, formation conditions. BO textures have been replicated using dynamic crystallization techniques on olivine-rich chondrule starting compositions in an attempt to determine these formation conditions. One composition was modeled after the average BO chondrule composition in L-chondrites determined by WEISBERG (1987) in a survey of BO chondrules. Chondrule droplets are presumed to form by the melting of crystalline material. BO textures developed best if the crystalline precursor was melted slightly above to tens of degrees above the liquidus temperature eliminating all ready nuclei. Crystalline material (embryos) of subcritical nuclei size in the melts provide the nuclei when they reach critical nucleus size at degrees of supercooling which stabilize the plate dendritic crystal form of olivine. The plate dendrite is the basic crystal unit of the BO texture. Multiple-plate dendrites are the common texture in experimentally produced chondrules; the "classic," single-plate-dendrite BO chondrule has not been reproduced in the experiments. While the preferred BO composition of Weisberg does produce BO textures over a broader range of melting temperatures, any composition that has olivine on the liquidus for a significant temperature interval will develop BO texture over some range of melting temperatures. The range of possible melting temperatures depends on the range of compositions available that have olivine on liquidus. The low end of the range will be near 1500°C and the upper end near 1700°C. The range of cooling rates that produced BO textures in the compositions studied is 500 to 2300°C/h.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- December 1990
- DOI:
- 10.1016/0016-7037(90)90303-3
- Bibcode:
- 1990GeCoA..54.3537L
- Keywords:
-
- Chondrule;
- Dendritic Crystals;
- Melts (Crystal Growth);
- Meteoritic Composition;
- Meteoritic Microstructures;
- Olivine;
- Crystal Structure;
- Solar Corona;
- Solar System;
- Textures