Sm-Nd isotopic evolution of chondrites and achondrites, II

Abstract

The 147Sm 143Nd and 146Sm 142Nd isotope systematics have been investigated in five chondrites and the achondrites Moama and Angra dos Reis (ADOR). The new chondrite data and those we have reported before are all consistent with our previously reported reference values for CHUR (“chondritic uniform reservoir”) of ( 143Nd/ 144Nd) CHUR 0 = 0.511847 and ( 147Sm/ 144Nd) CHUR 0 = 0.1967 . Most of the bulk chondrites analyzed have 143Nd/ 144Nd and 147Sm/ 144Nd within 0.5 ε-units and 0.15% of the CHUR values, respectively. This strongly suggests that the CHUR evolution is now known to within these error limits throughout the history of the solar system. The St. Severin chondrite yields an Sm Nd internal isochron age of T = 4.55 ± 0.33AE and an initial ε Nd = 0.11 ∓ 0.26 . Much larger variations in Sm/Nd ratios were measured in mineral separates of the Moama and ADOR achondrites. Thus, very precise ages of 4.46 ± 0.03AE and4.564 ± 0.037AE were obtained for these meteorites, respectively. The initial ε Nd values obtained for Moama and ADOR are 0.03 ∓ 0.25and0.14 ∓ 0.20, respectively. The values obtained on these meteorites are fully consistent with the CHUR evolution curve. Initial ε Nd data on terrestrial igneous and meta-igneous rocks demonstrates that positive initial ε Nd values occur throughout the past 4 AE. This confirms our earlier report that a light rare earth element-depleted layer has existed throughout most of the Earth history and is the source of present-day mid-ocean ridge basalts. The inferred shape of the ε Nd vs. age curve for the depleted mantle suggests profound changes in tectonic regimes with time; in particular, it suggests a much higher rate of recycling of continental materials into the mantle during the Archean as compared to later time periods. 146Sm 142Nd systematics of ADOR and Moama are supportive of the hypothesis that 146Sm was present in the early solar system and suggests a 146Sm/ 144Sm ratio of about 0.01 for the solar system ∼ 4.56 AE ago. This inferred high 146Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.