Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..501d&link_type=abstract
Meteoritics, vol. 30, no. 5, page 501
Other
1
Chondrules, Nonthermal, Nucleation, Supersaturation, Texture Development
Scientific paper
The textures of chondrules are almost universally ascribed to the influence of heating and cooling histories of the precursor materials. Many chondrule textures are viewed as constraining the cooling history of precursor droplets of silicate liquid that were initially heated to near liquidus temperatures. The standard thermal model uses a superliquidus silicate droplet that was significantly undercooled to induce nucleation and then cooled, possibly nonlinearly, down a thermal gradient to induce crystal growth in the chondrule. Alternately, nuclei may occur as relicts in the droplet or may be introduced as dust on the surface of the droplet. The means by which the nucleation barrier (the essential first step in the crystal growth history of a chondrule) is overcome has fundamental influence on the subsequent development and type of texture - the development of dendritic and porprhyritic textures reflect quite different nucleation histories. We explore an alternate method of 'jumping' this barrier. Undercooling or, more generally, supersaturation of the liquidus solid, can be induced either by cooling an isochemical system from superliquidus temperatures or by isothermally making the bulk composition of the system more refractory. Removal of some FeO from a liquid of typical chondrule bulk composition increases the liquidus temperature and supersaturates the liquid with the most refractory solid, olivine. FeO is removed easily by reduction to metal. Most chondrules are more magnesian than the solar system composition and indeed most are more magnesian than the host meteorite in which they occur. Clearly the source materials of chondrules have lost Fe relative to either bulk solar system or chondritic precursors. If reduction was involved in chondrule formation then Fe-loss would induce undercooling by forcing the chondrule bulk composition to become more magnesian. (If the Fe-loss occurred prior to the chondrule forming event, then isothermal supersaturation is less likely to occur.) Variability of the degree of Fe-loss dominates much of the compositional range of chondrules with Type I chondrules having the lowest Fe- contents. A plausible mechanism for reduction of the silicate droplets in a chondrule forming region, is gas expansion. During heating, the formation of silicate liquid droplets will occur at equilibrium with the "chondrule" vapor phase and any ambient gas. The vapor+gas cloud associated with the heating cycle of chondrule precursors usually will expand more rapidly than the 'cloud' of liquid droplets it contains. As it expands, the fugacity of most species in the gas cloud, including oxygen species, drops, initiating reduction of the entrained droplets. Surface area/volume considerations require that small droplets (<0.5mm) will react much more rapidly than large droplets (>1mm) and as a result a variety of size sorted compositional groups of chondrules may form. Since reduction is initiated on the outer surface of the droplet, simple diffusional arguments indicate that those outer surfaces will experience reduction induced supersaturation first and can precipitate metal and Mg-rich olivine nuclei from which the texture of the chondrule may grow as diffusion limited reduction proceeds inward. Metal grains may act as nucleation sites for olivine in the chondrule or may be lost from the chondrule because of the differential expansion of metal and silicate. The necessity that precipitation begin on the external surfaces of the droplet suggests that textures characterized by annular structures are likely to be formed by this process. The best example is, of course, classic barred olivine, a texture that is approximated by a variety of dendritic textures in cooling experiments. The production of the characteristic annular shell of olivine enclosing the bars in barred olivine chondrules, may be experimentally difficult in current furnaces but its absence in most experiments may also be the result of inappropriate conditions. The final growth of the chondrule texture must involve a suitable cooling path and reduction induced nucleation probably acts in tandem with thermal cycling of chondrule melt droplets.
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