Old Sm–Nd ages for cumulate eucrites and redetermination of the solar system initial 146Sm/ 144Sm ratio

Abstract

Short-lived 146Sm– 142Nd and long-lived 147Sm– 143Nd chronometers have been measured in three cumulate eucrites (Binda, Moore County and Moama). The two major mineral phases (plagioclase and pyroxene) present in these achondrites are characterized by a wide range of Sm/Nd ratios that allows well-resolved Sm–Nd isochrons. This group of meteorites thus is suitable to better constrain the initial 146Sm/ 144Sm ratio of the solar system. Binda and Moore County give concordant ages of 4544 ± 88 and 4542 ± 85 Ma, respectively, with initial 143Nd/ 144Nd ratios slightly higher, to within error, of chondritic. These ages are in agreement with most of the radiometric ages determined on basaltic eucrites. A best estimate for the solar system initial 146Sm/ 144Sm ratio is obtained using the five-point regression line determined for Binda. The 146Sm/ 144Sm ratio of 0.00728 ± 57 obtained for this sample translates to a 146Sm/ 144Sm ratio at 4568 Ga of 0.0085 considering the age of isotopic closure obtained from 147Sm– 143Nd systematics. When 146Sm– 142Nd data from the literature are examined in detail, four eucrites have concordant 147Sm– 143Nd and 146Sm– 142Nd systematics. Their weighted average 147Sm– 143Nd age is equal to 4546 ± 8 Ma. An initial 146Sm/ 144Sm ratio at 4568 Ma calculated from these samples is 0.0084 ± 0.0005. A similar ratio of 0.0085 ± 0.0007 is calculated if data from different groups of achondrites (angrite and mesosiderite) are included in the calculation. No difference in the 146Sm/ 144Sm ratios or initial 142Nd/ 144Nd ratios is observed among different groups of achondrites relative to ordinary chondrites. This work suggests that 146Sm was homogeneously distributed and that both Sm and Nd were isotopically uniform at the planetary scale in the solar system, at least in the region around where these planetary bodies formed.