Cat Mountain: A meteoritic sample of an impact‐melted asteroid regolith

Abstract

Cat Mountain is a new ordinary chondrite impact melt breccia that contains several shocked chondrule‐bearing clasts of L5 material. These clasts are surrounded by a total impact melt of similar composition material which appears to have cooled over a period of a few thousand years, probably within a melt breccia lens in the bottom of a large (>1 km diameter) crater on an L chondrite asteroid. Noble gas isotopes indicate that the sample was involved in at least two different impact events, approximately 880 and 20 Myr ago, following the 4.55 Ga accretion of primitive chondritic material. The 880 Ma event is responsible for the impact breccia texture of the sample, and the 20 Ma event reduced the sample to a meter‐sized object. We also infer that another impact occurred between 880 and 20 Ma (possibly the ∼500 Ma event recorded in many other L chondrites) to jettison the material from the asteroid belt into an orbit that evolved into an Earth‐crossing trajectory. The shock‐metamorphic processes that occurred at 880 Ma redistributed the opaque phases in the meteorite and altered the crystalline characteristics of silicate phases. This reduced the reflectance of the L5 material and decreased the amplitude of its spectral absorption features. These characteristics are consistent with the spectral characteristics of some C class asteroids and suggest that some dark asteroids that appear to belong to the C class could be covered with shocked ordinary chondrite material. If one assumes that Cat Mountain came from the same asteroid as other L chondrites with the same cosmic ray exposure age, then the juxtaposition of these different materials suggests asteroids are rubble piles which are heterogeneous on a scale less than 100 m. Furthermore, the structural integrity of Cat Mountain and other L chondrites suggests the strengths of asteroid rubble piles are limited by fractures and contrasting material properties and are thus inherently weak in a ram pressure regime produced when they enter a planetary atmosphere. However, in a regime where the asteroid is the target of impact fragmentation rather than the projectile, the added porosity of a rubble pile structure will compensate for the presence of fractures and absorb a large amount of the impact energy. In this case the structural integrity of the asteroid may appear to be the same as a previously unshocked chondritic material.

New eucrite Dar al Gani 872: Petrography, chemical composition, and evolution

“…The micro analyzer installed at the University of Arizona Lunar and Planetary Laboratory is a Cameca SX-50. The procedure applied for the investigation of DaG 872 has been described by Kring et al (1996). Details on the parameters and techniques involved with the INAA have been given by Hill et al (1991;see also Patzer et al 2001).…”

Analyses of the chondritic meteorite Orvinio (H6): Insight into the origins and evolution of shocked H chondrite material

“…Uranium/thorium-helium, K-Ar, and 40 Ar/ 39 Ar all indicate a major event affecting L chondrite material at ~500 Ma (Schmitz et al 2003;Bogard 1995;Haack et al 1996;Heymann 1967). Other clusters of ages have also been suggested at 40,350,880, and 2500 Ma (Alexeev 1995(Alexeev , 1998Kring et al 1996;Bogard et al 1976).…”

“…H chondrites do not record any obvious specific thermal impact events in the last 3 Ga using 40 Ar/ 39 Ar methodology (see Table 1 [Bogard 1995;Grier et al 1996;Kring et al 1996]). They do record a broad peak in U, Th-He ages at 3-4 Ga (Wasson and Wang 1991) and a cosmic ray exposure (CRE) age peak at ~7-8 Ma (Graf and Marti 1995), and an analysis of 3 He and 4 He suggests the possibility of local heating and degassing about 200 Ma ago (Alexeev 1998).…”

Composition of the first bulk melt sample from a volcanic region of Mars: Queen Alexandra Range 94201

“…Silicates and oxides were analyzed together, while sulfides were analyzed separately using different sets of standards. Additional details about standards, calibration, and analytical procedures are reported elsewhere (Kring et al 1996b). …”