The determination of the initial ¹⁴⁶Sm abundance of the outer Solar System

Abstract

Recently, an "isotopic dichotomy" has been identified between carbonaceous (CC) and non-carbonaceous (NC) meteorites. This dichotomy may reflect the location of formation of the meteorite parent bodies, indicating that CC and NC materials originated in the outer and inner regions of the Solar System, respectively. While many differentiated meteorites fall into the NC category, some unclassified achondrites exhibit CC-like isotopic compositions. As for Northwest Africa (NWA) 6693/6704, they are considered to be CC-like achondrites showing isotopic affinity to CR chondrites.The radionuclide ¹⁴⁶Sm has a half-life of 103 million years and undergoes decay to ¹⁴²Nd. The initial ¹⁴⁶Sm/¹⁴⁴Sm ratios are crucial for understanding the evolution of planetary materials in the early Solar System. These primary ratios have been determined from the calcium-aluminum-rich inclusions (CAIs) of the Allende meteorite and the oldest andesitic meteorite, Erg Chech 002. However, these ratios have not been measured in materials from the CC region. In this study, we conducted high-precision neodymium isotope ratio analyses of NWA 6693/6704 using TIMS. The obtained ¹⁴⁶Sm/¹⁴⁴Sm primary ratios align with those found in the NC region and in CAIs.