Shock metamorphism of ordinary chondrites

A revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed. Six stages of shock (S1 to S6) are defined, based on shock effects in olivine and plagioclase as recognized by thin section microscopy. The characteristic shock effects of each shock stage are: S1 (unshocked) - sharp optical extinction of olivine; S2 (very weakly shocked) - undulatory extinction of olivine; S3 (weakly shocked) - planar fractures in olivine; S4 (moderately shocked) - mosaicism in olivine; S5 (strongly shocked) - isotropization of plagioclase (maskelynite) and planar deformation features in olivine; and S6 (very strongly shocked) - recrystallization of olivine, sometimes combined with phase transformations (ringwoodite and/or phases produced by dissociation reactions). S6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked. In stages S3 to S6, localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure, due to differences in shock impedance. These melting effects are (a) opaque melt veins (shock veins); (b) melt pockets with interconnecting melt veins; (c) melt dikes; and (d) troilite/metal deposits in fractures. Based on a critical evaluation of data from shock recovery experiments, a shock pressure calibration for the six shock stages is proposed, which defines the S1 /S2, S2/ S3, S3/S4, S4/S5, and S5/S6 transitions at < 5, 5-10, 15-20, 30-35, and 45-55 GPa, respectively. Whole-rock melting and formation of impact melt rocks or melt breccias occurs at about 75-90 GPa. The symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite, e.g., H5(S3). We propose this new shock classification and pressure calibration system to replace previous systems, which are out-of-date with respect to the pressure calibration, nominally restricted to L chondrites, and based on incomplete and, in part, illdefined sets of shock effects. We have classified seventy-six ordinary chondrites using the new classification system and conclude the following: 1) Shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor; e.g., L chondrites appear to have the largest fraction with stages S5 and S6. This suggests that the collisional histories of the H, L, and LL parent bodies were similar.

2 ) Petrologic type 3 chondrites are deficient in stages S4 to S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. We suggest that the more porous and volatile-rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Volatiles trapped in pores cause shock-induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites.

3 ) Stage S3 is the most abundant in nearly all petrologic types.

4) At shock pressures in excess of about 35 GPa (S5 and S6), ⁴He and ⁴⁰Ar are almost completely lost; pressures below 10 GPa (S1 and S2) do not cause noble gas losses.