¹Ryuji Okazaki, ^2,3Keisuke Nagao
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12819]
¹Faculty of Sciences, Department of Earth and Planetary Sciences, Kyushu University, Nishi-ku, Fukuoka, Japan
²Graduate School of Science, Geochemical Research Center, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
³Korea Polar Research Institute, Incheon, Yeonsu-gu, South Korea
Published by arrangement with John Wiley & Sons

The Sutter’s Mill (SM) CM chondrite fell in California in 2012. The CM chondrite group is one of the most primitive, consisting of unequilibrated minerals, but some of them have experienced complex processes occurring on their parent body, such as aqueous alteration, thermal metamorphism, brecciation, and solar wind implantation. We have determined noble gas concentrations and isotopic compositions for SM samples using a stepped heating gas extraction method, in addition to mineralogical observation of the specimens. The primordial noble gas abundances, especially the P3 component trapped in presolar diamonds, confirm the classification of SM as a CM chondrite. The mineralogical features of SM indicate that it experienced mild thermal alteration after aqueous alteration. The heating temperature is estimated to be 500 °C. The variation in the heating temperature of thermal alteration is consistent with the texture as a breccia. The heterogeneous distribution of solar wind noble gases is also consistent with it. The cosmic-ray exposure (CRE) age for SM is calculated to be 0.059 ± 0.023 Myr based on cosmogenic 21Ne by considering trapped noble gases as solar wind, the terrestrial atmosphere, P1 (or Q), P3, A2, and G components. The CRE age lies at the shorter end of the CRE age distribution of the CM chondrite group.

^1,2,3Tomáš Kohout et al (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12871]
¹Department of Physics, University of Helsinki, Finland
²Institute of Geology, The Czech Academy of Sciences, Praha, Czech Republic
³Finnish Fireball Network, Finland
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar-Ar age of 4.4 Ga. Its cosmic ray exposure history is atypical as it is not part of the large group of H chondrites with a prominent 7–8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 ± 4 Ma. The results from short-lived radionuclides are compatible with an atmospheric pre-entry radius of 30–40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The 10Be concentration indicates a recent (3–5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30–35 cm pre-entry radius.