HIGH FLUENCES OF NEUTRONS DETERMINED FROM SM AND GD ISOTOPIC COMPOSITIONS IN AUBRITES

Abstract

Large isotopic deviations of 149Sm-150Sm and 157Gd-158Gd derived from neutron capture effects were observed in five enstatite achondrites (aubrites); Norton County, ALH78113, Cumberland Falls, Bishopville and Mayo Belwa. The isotopic variations of these aubrites (#149Sm = -32 to -7, #150Sm = +11 to +57, #157Gd = -42 to -14 and #158Gd = +9 to +26) correspond to irradiation with neutron fluence of (1.17-3.99)x1016 n cm-2, which is within the range of estimated neutron fluence values of some lunar samples. These results indicate that aubrites may have been located near the surface of their parent body for several hundreds of Ma. Furthermore, the neutron energy spectrum obtained from the combination of Sm and Gd isotope systematics (#Sm/#Gd) suggests that the energy of thermalized neutrons in five aubrites was in the range of 200-400 K, and was lower than those in lunar samples A-12 and A-15. In addition, we analyzed one anomalous aubrite (Mt. Egerton), and two enstatite chondrites (Happy Canyon and Abee) to investigate the relationship of their origin with aubrites. However, these three meteorites showed no evidence for intensive neutron irradiation observed in the aubrites.