Metallic minerals, thermal histories and parent bodies of some xenolithic, ordinary chondrite meteorites

Complex Thermal Histories of L Melt Breccias NWA 5964 and NWA 6580

“…This situation is analogous to that found by Scott and Rajan (1981) in some genomict ordinary chondrite breccias.…”

“…(Wood, 1988;Hutchison, 2004;Bischoff et al, 2006). Many chondrites fall into the category of genomict breccias, containing clasts of similar composition but various petrographic types (metamorphic grades) (Dodd, 1981;Scott and Rajan, 1981;Wood, 1988;Hutchison, 2004;Bischoff et al, 2006;Weisberg et al, 2006). Few chondrites are polymict breccias, containing clasts of two or more unique compositions; while some are composed of shock lithified fragments from the surface of a planetesimal, either regolith breccias that show direct evidence for surface exposure in the form of implanted solar wind gases or flare tracks, or fragmental breccias that lack surface exposure evidence but that consist of a variety of debris (Wood, 1988, Housen andWilkening 1978).…”

Equilibrated ordinary chondrites: Constraints for thermal history from iron‐magnesium ordering in orthopyroxene

“…Catastrophic processes that are able to account for our data might be barely shattering or cone-shattering models (Fujiwara et al, 1989). Alternative %The lack of correlation between petrographic types and cooling rates at -340-480 "C may be consistent with the rubble-pile models (e.g., Scott and Rajan, 1981;Grimm, 1985;Taylor et al 1987). In these models, peak metamorphism was reached in bodies that, after low-velocity mutual collisions, gravitationally and chaotically reassembled into mega-breccias before final cooling at <-500 "C. Miyamoto and Fujii, 1980;Pellas and Storzer, 1981;Pellas and Fieni, 1988), the petrographic types call for a layered structure with distinct thermal regimes when at 7480 "C. The similar cooling rate ranges recorded by types 4 to 6 require isogrades independent of petrographic type when at 340-480 "C. This feature does not fit the conventional onion-shell thermal models that predict correlations between petrographic types and cooling rates according to depths the types cooled in.…”

39Ar‐40Ar chronology of R chondrites

“…Several lines of evidence suggest that ordinary chondrites (H, L, and LL) experienced a period of extended internal metamorphism following formation of their parent bodies. This evidence includes 39 Ar ages of 4.52-4.38 Ga (Turner, Enright, and Cadogan 1978), Pb-Pb ages of whole rock and phosphate samples of 4.41-4.56 Ga, Pu fission track ages down to ~4.4 Ga (Pellas and Fieni 1988), and Ni metallographic cooling rates as low as a few °C/Myr (Scott and Rajan 1981). In contrast, Ar-Ar ages of shocked ordinary chondrites are generally <4.2 Ga, though a few ages (such as Shaw) that lie between 4.3 and 4.54 Ga may have been produced by early impacts (Bogard 1995;Bogard and Hirsch 1980;Taylor et al 1979;Turner 1969).…”