Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Harry Y. McSween

Abstract

Chondrite groups have distinct size-frequency distributions of chondrules, commonly attributed to mass or aerodynamic sorting in the solar nebula. The concept of sorting has also been extended to other chondrite constituents (metal chondrules and metal-troilite grains), which have similar size-frequency distributions but smaller mean sizes. The different mean sizes of the chondrules and metal-troilite grains have been attributed to their varying densities in support of the concept of sorting, according to which the two populations should be mass equivalent. However, mass equivalence has never been demonstrated for chondrules and metal-troilite grains in the same meteorite.

I present the results of a comparison of chondrules and metal-troilite grains from three ordinary chondrites: Kelly (LL4), Bjurbole (L/LL4), and Hammond Downs (H4). The two populations are not mass equivalent in these three meteorites, and therefore do not provide compelling evidence for nebular sorting. However, trends in my data suggest that sorting did occur, but was inefficient, and imply that the efficiency of sorting varied between the populations, and also between the ordinary chondrite groups.

In each meteorite the average chondrule outweighs the average metal-troilite grain, but the two populations are the most similar in Hammond Downs. The average masses become increasing dissimilar in Bjurbole and Kelly, the average chondrule becoming heavier and the average metal-troilite grain lighter. The observed range in mass of the chondrules is narrower than that of the corresponding metal-troilite grain populations in each sample, suggesting that the chondrules were more efficiently sorted (2σ for the chondrules in each meteorite: Kelly, +0.93mg, -0.41mg; Bjurbole, +0.73mg, -0.31mg; Hammond Downs, +0.28mg, -0.13mg; 2σ for the metal-troilite grains: Kelly, ±0.91mg; Bjurbole, ±0.43mg; Hammond Downs, ±0.34mg). The sorting efficiency also appears to vary between the ordinary chondrites, the mass distributions of both populations being relatively narrow in Hammond Downs and increasing in Bjurbole and Kelly.

Because they are not mass equivalent my data fit aerodynamic sorting models better than those which infer sorting by mass alone. Aerodynamic stopping times were found to increase with particle size and mass between the three meteorites. Because of this, the aerodynamic stopping times of the two populations are the most similar in Hammond Downs and the most disparate in Kelly. Significantly, the mean chondrule and metal-troilite grain diameters in Hammond Downs most closely approximate the size of particles which Cuzzi et al. (1996) argue would be the best sorted. The relatively poor sorting of the metal-troilite grains may result from their irregular shapes. The greater abundance of small grains may have also influenced the sorting efficiency of this population because small grains are more easily mobilized by turbulence than large particles.

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