Masters Theses

Date of Award

6-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Harry Y. McSween

Committee Members

Nick Woodward

Abstract

Deformation of chondrules and development of a foliation have been observed In several carbonaceous chondrites, of which the Leoville CV3 chondrite (an accretionary breccia) displays the most evident deformation. Leoville Is especially well suited for strain analysis and study of deformation because the original meteorite owner sliced It Into large planar sections which show large axial ratios. It Is unlikely that much greater axial ratios existed without being reported, so the cuts fortuitously roughly parallel the λ 1 ~λ 2 (maximum and minimum) plane. An aspect ratio of 1.9:1 Is Indicated for strain In the chondrules alone, whereas a value of 2.1:1 Is Indicated for the whole chondrite, corresponding to between 31 and 34% uniaxial shortening. At geologically reasonable terrestrial strain rates of 10 -13 to 10-15/s, this much shortening would require approximately 10 6 to 10 8 years. Plastic deformation by diffusional flow or grain boundary sliding are reasonable mechanisms of deformation based on thin section analysis. Shock and metamorphism are ruled out as likely mechanisms. Compaction due to overburden with progressive accretion on the chondrite parent body Is the probable cause of deformation.

Modeling deviatoric stress for an asteroidal body between 150 and 300 km In radius suggests that Leoville experienced between 0.09 kb and 0.75 kb deformational pressure. Maximum temperatures for CV3 chondrites were between 500 and 600°C based on experimental data. Deformation of the Leoville parent body by diffusional flow Is possible with these pressures and temperatures but only at high strain rates.

strain rates of 10 -13 to 10-15/s require 700 to 1000°C. It Is unlikely that Leoville experienced temperatures this high. Perhaps strain rates were higher, or the deformation more complex. Intergranular water. If present, may have Increased the strain rate at lower temperatures.

Analysis of three other CV3 chondrites (Ball, Grossnaja, and Efromovka) Indicates that they experienced deformation similar to Leoville. This Implies that this type of deformation was a result of asteroidal body Interior processes rather than Isolated surface Inci dents such as Impact. Such Information about asteroidal Interiors provides an independent means of assessing their character.

If metallographic cooling rates In Leoville and other chondrites are found to correlate with foliation, magnetic anisotropy, and compaction, this may support the rubble-pile parent body model. Conversely, lack of such correlations may provide evidence against the rubble-pile means of formation.

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