Sm and Gd Isotopic Shifts in Eucrites and Implications for Their Cosmic-Ray Exposure History

Abstract

The isotopic compositions of Sm and Gd in eight eucrites—five from a desert, Dar al Gani (DaG) 380, DaG 391, DaG 411, DaG 443, and DaG 480, and three from nondesert areas, Juvinas, Millibillillie, and Stannern—were determined to understand the cosmic-ray exposure (CRE) history for each meteorite from the isotopic shifts of 149Sm–150Sm and 157Gd–158Gd caused by the neutron capture reactions induced by cosmic-ray irradiation. Seven of the eight samples, excepting DaG 443, show readily detectable isotopic shifts of Sm and Gd corresponding to neutron fluences in the range of (0.28–2.38) × 1015 neutrons cm−2. The degrees of Sm isotopic shifts for six of these seven eucrites can be consistently explained by the CRE age histogram of eucrites obtained in previous studies. Exceptionally, DaG 480 shows larger isotopic shifts of Sm than those expected from the CRE age histogram, suggesting a multiple-irradiation history, including irradiation on the parent body. However, there is no clear difference in the CRE conditions between DaG 480 and other eucrites from the parameter ε Sm/ε Gd to identify the difference in the thermalization degree of neutrons in association with the CRE conditions.