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

Abstract

Abstract-We have studied the petrography, reflectance spectra, and Ar-Ar systematics of the Orivinio meteorite. Orvinio is an H chondrite not an L chondrite as sometimes reported. The material in the meteorite was involved in several impact events. One impact event produced large swaths of impact melt from H chondrite material surrounding relict clasts of chondrule-bearing material. Not only were portions of a bulk H chondrite planestesimal melted during the impact event, but shock redistribution of metal and sulfide phases in the meteorite dramatically altered its reflectance spectra. Both the melt and relict clasts are darker than unshocked H chondrite material, bearing spectral similarities to some C-class asteroids. Such shock metamorphism, which lowers the albedo of an object without increasing its spectral slope, may partially explain some of the variation among S-class asteroids and some of the trends seen on asteroid 433 Eros. Noble gases record the evidence of at least two, and perhaps three, impact events in the meteorite and its predecessor rocks. The most significant evidence is for an event that occurred 600 Ma ago or less, perhaps ~325 Ma ago or less. There is also a signature of 4.2 Ga in the Ar-Ar systematics, which could either reflect complete degassing of the rock at that time or partial degassing of even the most retentive sites in the more recent event.

Solar System Time Scales from Long-Lived Radioisotopes in Meteorites and Planetary Materials

“…The timing of impact events affecting chondrite parent bodies has been determined predominantly with the 39 Ar-40 Ar dating method (e.g., Bogard and Hirsch, 1980;Bogard et al, 1976;Grier et al, 2004;Kaneoka, 1981;Kring et al, 1996;Stephan and Jessberger, 1988;Turner, 1969), although in some cases other isotope systems such as Rb-Sr, Pb-Pb, and Sm-Nd (roughly in that order of susceptibility to disturbance) are also affected by such events. Most experimental work on the effects of shock alone on various isotopic chronometers indicates that shock pressures up to $60 GPa are usually insufficient to reset these chronometers (Deutsch and Schärer, 1994).…”

On the origin of shocked and unshocked CM clasts in H‐chondrite regolith breccias

“…While the timing of the H-chondrite-family-forming event is unknown, the H chondrites were affected by numerous Ar-Ar shock-degassing events 3.4-4.1 Ga ago as well as within the last 600 Ma. Overall, the most prominent Ar-Ar features occurred within the last 250-325 Ma (Bogard 1995;Grier et al 2004;Swindle et al 2009). While we do not know if the 250-325 Ma and 3.4-4.1 Ga events produced any asteroid families, impact events energetic enough to produce Ar-Ar shock degassing events should also have produced a moderate amount of ejecta.…”

Raman spectroscopic study of four Spanish shocked ordinary chondrites: Cañellas, Olmedilla de Alarcón, Reliegos and Olivenza

“…Anders 1964;Keil 1964;Heymann 1967;Neal et al 1981;Walsh & Lipschutz 1982;Walsh et al 1983;Friedrich et al 2001Friedrich et al , 2002Friedrich et al , 2008Grier et al 2004;Horz et al 2005;Friedrich 2006;Ferroir et al 2008;Hirata et al 2009). Consequently, prior to a chemical study, it is convenient to determine whether the chondrite has been affected by shock, and to what extent.…”