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

Short-lived Sm-146-Nd-112 and long-lived Sm-147-Nd-143 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 Sm-146/Sm-144 ratio of the solar system. Binda and Moore County give concordant ages of 4544 +/- 88 and 4542 +/- 85 Ma, respectively, with initial Nd-143/Nd-144 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 Sm-146/Sm-144, ratio is obtained using the five-point regression line determined for Binda. The Sm-146/Sm-144 ratio of 0.00728 +/- 57 obtained for this sample translates to a Sm-146/Sm-144 ratio at 4568 Ga of 0.0085 considering the age of isotopic closure obtained from Sm-147-Nd-143 systematics. When Sm-146-Nd-142 data from the literature are examined in detail, four eucrites have concordant Sm-147-Nd-143 and Sm-146-Nd-142 systematics. Their weighted average Sm-147-Nd-143 age is equal to 4546 +/- 8 Ma. An initial Sm-146/Sm-144 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 Sm-146/Sm-144 ratios or initial Nd-142/Nd-144 ratios is observed among different groups of achondrites relative to ordinary chondrites. This work suggests that Sm-146 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. (c) 2010 Elsevier B.V. All rights reserved.