Deformation of ordinary chondritic meteorites

Chondrule strain and magnetic anisotropy are independent measurements of deformation. For the first time, three-dimensional analyses of strain in chondrules and magnetic susceptibility measurements have been performed on the same ordinary chondritic meteorites. These data reveal direct correlations in shape and orientation of the two strain indicators.

Employing computerized strain analysis techniques, a continuous gradation of strain in chondrules was discovered in fifteen ordinary chondrites. Values of strain range from a maximum in Mezo-Madaras of 17% flattening, with mean ellipsoid axial ratios o 1.15;1.00;0.SO, to a minimum strain in Kelly of 3% flattening and axial magnitudes of 1.02:1.00:0.96. The mean ellipsoids generated from the samples predominantly have flattening-type or "pancake" spheroidal shapes and are aligned to produce foliations.

Magnetic susceptibility anisotropy was measured for eight of the samples in which the strain was analyzed and magnitudes of the principal axes of the susceptibility ellipsoids were determined. The susceptibility ellipsoids generated for each sample also have pancake spheroidal shapes. Gradation in degree of magnetic anisotropy development was discovered to be directly proportional to gradation of chondrule strain in the samples. In addition to correspondence in ellipsoid shape, orientation of the c (shortened) axis of the strain ellipsoid aligned subparallel to the XI (shortened) axis of the magnetic susceptibility ellipsoid in six of the eight samples measured. A range in the degree of shortening in ordinary chondrites is indicated by these new strain and magnetic measurements. The shapes the of strain and susceptibility ellipsoids are consistent with the hypothesis that deformation was caused by shortening along the principal minimum axes.

Chondrule deformation was discovered to be due to shock processes using several methods. First, each sample was assigned to a shock facies, based on optical criteria, and plotted against its respective percent shortening. These two variables were shown to correlate. Second, noble gas concentrations were noted to have been depleted with increasing strain. Because depletion of noble gas concentrations is also a function of shock, it is assumed that the strain measured was shock-induced. Transmission electron microscopy analyses of a high-strain and a low-strain sample allowed observation of substructures in olivine grains. The density and distribution of dislocations discovered in both samples provided further evidence for shock deformation. Based on these three shock indicators, it is believed that the chondrule strains and magnetic anisotropies quantified and correlated in this study were formed by dynamic compaction during impacts on chondrite parent bodies.