Neutron capture records of mesosiderites and an iron meteorite

The Sm and Gd isotopic compositions of silicates from six mesosiderites (Dalgaranga, Estherville, Morristown, Northwest Africa (NWA) 1242, NWA 2932, and Vaca Muerta) and one iron meteorite (Udei Station) were determined to elucidate the cosmic-ray exposure records. All seven samples showed significant 150 Sm/ 149 Sm and 158 Gd/ 157 Gd isotopic shifts from neutron capture reactions corresponding to neutron fluences of (1.3–21.8) × 10 15 n cm −2 . In particular, Vaca Muerta showed a significantly higher neutron fluences than the other six samples. The parameter for the degree of neutron thermalization ( e Sm / e Gd ) also showed a significant difference between Vaca Muerta (0.76) and the other samples (0.93–1.20). These results suggest a two-stage irradiation of the Vaca Muerta silicates in the parent body (>50 Ma) before formation of the mesosiderite and during its transit to Earth (138 Ma). This is consistent with the 81 Kr–Kr cosmic-ray exposure age data of a Vaca Muerta pebble from a previous noble gas isotopic study.