Shock metamorphism of carbonaceous chondrites

Abstract

We have studied shock effects in carbonaceous chondrites using optical microscopy of thin sections and find that our petrographic classification of progressive shock metamorphism in ordinary chondrites can also be applied to carbonaceous chondrites. We find that sixty-nine carbonaceous chondrites can be assigned to four shock stages, SI to S4, largely on the basis of shock effects in olivine. The least shocked chondrite groups are CM2 and CO3: thirty-six out of thirty-eight members are classified as shock stage S1 (<5 GPa), the remainder are stage S2 or S3. The most strongly shocked groups are CK4-6 and CV3, which both have more than half their members in shock stages S2 and S3, the remainder are mostly stage S1. Two carbonaceous chondrites of shock stage S4 were found and none of stages S5 and S6. Opaque shock veins and melt pockets were observed in the two stage S4 chondrites, Efremovka (CV3) and EET 83311 (CK5), and a stage S3 chondrite, LEW 87009 (CK6): this is consistent with the distribution of impact melt in ordinary chondrites. Some carbonaceous chondrites appear to be breccias of material shocked to diverse shock stages prior to assembly of the material. For example, Murchison (CM2) appears to be a mixture of stage S1 and S2 material. Differences between the mean shock levels of certain groups of chondrites, e.g., CO3 and CV3, are probably due to stochastic differences in the sampling of their parent bodies. However, there is an overall tendency for the mean shock level of carbonaceous chondrites (and ordinary chondrites) to increase with increasing petrologic type that may reflect real differences in the shock level of near-surface materials on their parent bodies caused by intrinsic variations in the chemical and physical properties of these materials. Petrologic type 2 and 3 chondrites are more porous and richer in volatiles than types 4 to 6. The greater porosity of types 2–3 causes higher post-shock temperatures and melting at lower pressures, and the higher volatile contents ensure that strongly shocked material is more readily dispersed on release from high pressure. We suggest that type 2–3 material shocked above 20–30 GPa normally escapes from the parent asteroids and forms particles that are too small to survive as meteorites. In the CV3 group, there is a correlation between the degree of chondrule flattening and the intensity of shock metamorphism, analogous to that discovered in ordinary chondrites by Sneyd et al. (1988), suggesting that shock rather than static overburden pressure is responsible for chondrule flattening. We infer that the correlations are due to collapse of pores under shock pressures of at least 5–10 GPa and that shock is an important process affecting many physical properties of type 2–4 chondrites.