IGPP Seminar Series

Comparative Chondrule-ology: Physical Properties Produced by Multiple Melting in Dusty Districts of the Nascent Nebula

by Alan Rubin


The physical properties of chondrules reflect the nebular environments in which they formed. Chondrite groups with large average chondrule sizes have few radial pyroxene (RP) and cryptocrystalline (C) chondrules, numerous enveloping (i.e., nested) compound chondrules, many chondrules with igneous rims (which tend to be relatively thick), and relatively few sulfide-bearing low-FeO porphyritic chondrules. Chondrite groups with large chondrules have relatively low Na/Mg abundance ratios, reflecting volatilization during chondrule formation. Chondrite groups with small average chondrule sizes have the opposite properties. Enveloping compound chondrules and chondrules with igneous rims form a continuum and indicate that a remelting event occurred after the primary chondrule was incorporated into a dustball; in those instances, the dustball melted and the primary chondrule remained intact. Repeated episodes of extensive remelting after chondrules were surrounded by dust would tend to produce large chondrules. RP and C chondrules formed by complete melting of their precursor assemblages; remelting of RP and C chondrules that were surrounded by dust would tend to produce porphyritic chondrules as numerous small dust particles mixed with the melt and provided nuclei for crystallizing phenocrysts. This process would tend to diminish the numbers of RP and C chondrules. Correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust-rich mantles; these mantles were formed in locally dusty nebular environments. If chondrites are arranged in order of plausible formation distances from the Sun, the inferred nebular dust distribution resembles those of some protoplanetary disks around solar-mass T Tauri stars. Chondrite groups inferred to have formed near the Sun have fewer CAIs than those inferred to have formed farther away. The mechanism that cleared the dust from the inner solar system (possibly the formation of a giant planet) may also have been responsible for expelling the vast majority of CAIs.
Tuesday, 04 May 2010
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM