Mineralogical and chemical composition and cosmic‐ray exposure history of two mesosiderites and two iron meteorites

Abstract

Abstract-We performed a comprehensive study of the noble gas isotopic abundances, radionuclide activities, and mineralogical and chemical composition of two mesosiderites and two iron meteorites. For the mesosiderites Dong Ujimqin Qi and Weiyuan, the silicate and the metal phases were studied. The anomalous ataxite Rafriiti is not chemically related to any other meteorite class, whereas Ningbo is a type IVA octahedrite. The mineralogy and major and trace element abundances of the silicate phases of Dong Ujimqin Qi and Weiyuan are similar to those of other mesosiderites and distinct from those of the howardites. The cosmic-ray exposure history was studied based on the concentrations of the cosmogenic noble gas nuclei and radionuclide activities. For the iron meteorites, cosmic-ray exposure ages were calculated from the pairs 10Be-21Ne1 26A1-21Ne, and 36C1-36Ar. Ramti yields the youngest exposure age of all ataxites (6.8 ? 1.7 Ma), whereas that of Ningbo with 107 2 15 Ma falls within the range observed for the other octahedrites. The parent body break-up times of the mesosiderites Dong Ujimqin Qi and Weiyuan are 252 ? 50 and 25.9 ? 5.0 Ma, respectively. We find no evidence for a common break-up event for the mesosiderites and the howardites.

Cosmic-Ray Exposure Ages of Meteorites

“…Mesosiderites Figure 15 CRE ages of mesosiderites from Terribilini et al (2000c). Albrecht et al (2000), Welten et al (2001b).…”

“…The age distribution does not have any strong clusters, although Welten et al (2001b) note that three of 19 mesosiderites (Chinguetti, Crab Orchard, and Estherville) have CRE ages close to 70 Ma. Terribilini et al (2000c) call attention to the dissimilarity of the CRE age distributions for mesosiderites and HED meteorites.…”

The Twannberg (Switzerland) IIG iron meteorites: Mineralogy, chemistry, and CRE ages

“…Assuming a constant cosmic ray flux, the 36 (Eugster 2003). Their CRE ages are in a range of about 7 Ma for the Rafrüti anomalous ataxite (Terribilini et al 2000) to about 1000 Ma (cf. Wieler 2002).…”

Neutron capture records of mesosiderites and an iron meteorite

“…The CRE ages of mesosiderites are intermediate between those of stony and iron meteorites, and vary from <10 to 340 Ma (Terribilini et al, 2000), and they are long enough to study their exposure history from the accumulation of the cosmogenic products (Begemann et al, 1976;Albrecht et al, 2000;Welten et al, 2001). Furthermore, it is interesting to experimentally obtain the variation in the neutron production rate in a target with a chemical composition in stony and iron meteorites, because not much is known about the neutron production rates in mesosiderites.…”