D. Stoffler

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 We propose this new shock classification and pressure calibration system to replace previous systems, which are out-of-date with respect to the pressure calibration, nominally restricted to L chondrites, and based on incomplete and, in part, illdefined sets of shock effects. n nWe have classified seventy-six ordinary chondrites using the new classification system and conclude the following: n1. n1) 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. n n2. n2 ) 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. n n3. n3 ) Stage S3 is the most abundant in nearly all petrologic types. n n4. n4) 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.

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 ( 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.

Lithification of gas-rich chondrite regolith breccias by grain boundary and localized shock melting

We studied the fine-grained matrices (< 150 μm) of 14 gas-rich ordinary chondrite regolith breccias in an attempt to decipher the nature of the lithification process that converted loose regolith material into consolidated breccias. We find that there is a continuous gradation in matrix textures from nearly completely clastic (class A) to highly cemented (class C) breccias in which the remaining clasts are completely surrounded by interstitial, shock-melted material. We conclude that this interstitial material formed by shock melting in the porous regolith. In general, the abundances of solar-wind-implanted 4He and 20Ne are inversely correlated with the abundance of interstitial, shock-melted, feldspathic material. Chondrites with the highest abundance of interstitial, melted material (class C) experienced the highest shock pressures and temperatures and suffered the most extensive degassing. It is this interstitial, feldspathic melt that lithifies and cements the breccias together; those breccias with very little interstitial melt (class A) are the most porous and least consolidated.

The case for a younger Imbrium basin: New 40Ar-39Ar ages of Apollo 14 rocks

Petrographically and chemically well-characterized rocks of the Apollo 14 landing site (12 impact melt breccias, 3 clast-free impact melt rocks, 2 granulitic rocks, 1 cataclastic noritic anorthosite, 1 shocked gabbronorite, 1 mare basalt, and 1 plagioclase single crystal) have been dated by the 40Ar-39Ar technique. The samples represent two different formations at the Apollo 14 site: (1) small lithic clasts (10 specimens) from fragmental breccia 14063 ejected from the 26 Ma old Cone crater and one clast (mare basalt) from Cone crater soil 14140, and (2) large rock samples (10 specimens) collected throughout the regolith of the landing site. The first group of samples has exposure ages close to 26 Ma and 40Ar-39Ar ages ranging from 3.86 to 4.09 Ga, which indicates that breccia 14063 was assembled 3.86 Ga ago. The second group of samples displays lower 40Ar-39Ar ages ranging from 3.73 to 3.85 Ga. We interpret breccia 14063 as part of a Nectarian megabreccia unit which was incorporated into the Fra Mauro Formation as a megablock by secondary mass wasting of the ejecta blanket of a local pre-Imbrian crater. The remaining samples are thought to represent clasts of the Apollo 14 subregolith basement megabreccia defined as the Fra Mauro Formation. Consequently, the reported ages of these clasts confirm previous suggestions that the Fra Mauro Formation and therefore the Imbrium basin is only 3.75 Ga old and not 3.85 Ga as commonly assumed.

Loss of radiogenic argon from shocked granitic clasts in suevite deposits from the Ries Crater

Five granitic clasts from the Otting and Aumuhle quarries in the suevite ejecta deposits beyond the rim of the Ries, Germany, impact crater have been characterized as to modal composition, degree of shock, and loss of radiogenic argon. The shock pressures experienced by this selected suite of samples range from 60 GPa, and the samples have been modestly shocked to heavily shocked, or even melted. Ages for these samples were determined by the 39Ar40Ar technique. The least shocked specimen (10–15 GPa) shows very little loss of radiogenic Ar relative to the approximately 320 My age of the Variscan basement at the Ries. All other samples (~28 GPa, ~42 GPa, ~52 GPa, and an impact melt) suggest complete to nearly complete loss of radiogenic Ar at the time of the 15 My old impact event. Most of the radiogenic Ar in these granitic samples is contained in feldspar, with a lesser amount in biotite. Diffusive loss of Ar occurs relatively easily from these highly disordered feldspars. The essentially complete loss of radiogenic Ar from 4 of the 5 samples is, in large part, ascribed to thermal activation during the cooling history of the suevite rather than during initial passage of the shock wave. These data are consistent with the conclusion from previous paleomagnetic studies that the suevite was deposited at temperatures above 500°C. The fact that Staudacheret al. (1982) found almost no loss of radiogenic Ar in a wide variety of Ries samples is ascribed to two causes: a) most of their samples were shocked to < 15 GPa and were collected in drill cores from the modestly shocked crater bottom, which was not as hot as the suevite; and b) their observations were primarily made on hornblende and biotite separates that are more resistant to shock effects and to diffusive loss of Ar, compared to shocked feldspars. Loss of argon from our suevite samples likely occurred in a “hot”, post-impact ejecta layer. From data presented here we estimate that < 50% of the material composing the “hot” Ries ejecta should show at least partially reset K-Ar ages; such materials compose no more than 5% of the total displaced crater volume.