Neutron capture records of mesosiderites and an iron meteorite

摘要

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)ncm~(-2). In particular, Vaca Muerta showed a significantly higher neutron fluences than the other six samples. The parameter for the degree of neutron thermalization (ε_(Sm)/ε_(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 (>50Ma) before formation of the mesosiderite and during its transit to Earth (138Ma). 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.