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

a r t i c l e i n f o Keywords: 146 Sm– 142 Nd systematics eucrites early planetary differentiation isotope distribution through solar system Short-lived 146 Sm– 142 Nd and long-lived 147 Sm– 143 Nd 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 146 Sm/ 144 Sm ratio of the solar system. Binda and Moore County give concordant ages of 4544 ± 88 and 4542 ± 85 Ma, respectively, with initial 143 Nd/ 144 Nd 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 146 Sm/ 144 Sm ratio is obtained using the five-point regression line determined for Binda. The 146 Sm/ 144 Sm ratio of 0.00728 ± 57 obtained for this sample translates to a 146 Sm/ 144 Sm ratio at 4568 Ga of 0.0085 considering the age of isotopic closure obtained from 147 Sm– 143 Nd systematics. When 146 Sm– 142 Nd data from the literature are examined in detail, four eucrites have concordant 147 Sm– 143 Nd and 146 Sm– 142 Nd systematics. Their weighted average 147 Sm– 143 Nd age is equal to 4546 ± 8 Ma. An initial 146 Sm/ 144 Sm 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 146 Sm/ 144 Sm ratios or initial 142 Nd/ 144 Nd ratios is observed among different groups of achondrites relative to ordinary chondrites. This work suggests that 146 Sm 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. The early history of solar system evolution from planetesimal formation to initial differentiation can be studied using short-lived radioisotope systems to provide high temporal resolution. However, extinct radionuclides must be used in combination with long-lived chronometers in order to convert the relative time intervals provided by …