Isotopic ratios of Gd, Sm and Eu in “Abee” enstatite chondrite

Abstract This paper presents measurements of the isotopic ratios of Gd, Sm and Eu from the enstatite chondrite Abee, Because the oxidation state of enstatite chondrites is significantly less than that of other chondrites and of the Earth, Miyashiro has postulated that this class of meteorite originated closer to the Sun (although its recent orbit appears to have been significantly beyond 1 A). Depending on the model used for nucleosynthesis, the greater flux of secondary thermal neutrons resulting from proximity to the sun might be observed as isotopic anomalies in these elements, some isotopes of which have very large thermal neutron cross sections. The meteorite Abee was compared with two terrestrial samples, from North and from South America. The isotope ratios Gd 155 Gd 156 , Gd 157 Gd 158 , Sm 149 Sm 150 and Eu 151 Eu 153 are identical for the three samples studied, within maximum experimental uncertainties (i.e. 2-sigma) of 0·15, 0·1, 0·3 and 0·3 per cent, respectively. The uncertainties include possible sources of systematic error as well as statistical errors. On the basis of these results, we conclude that the differential thermal neutron irradiation between Abee and the earth does not exceed 3 × 10 15 neutrons/ cm 2 , for a uniform irradiation model, or that the dilution factor differed by less than 1·2 per cent for the FGH model.