Sm-Nd isotopic evolution of chondrites and achondrites, II

Abstract

The ^(147)Sm-^(143)Nd and ^(146)Sm-^(142)Nd 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 (^(143)Nd/^(144)Nd)_(CHUR)^0 = 0.511847 and (^(147)Sm/^(144)Nd)_(CHUR)^0 = 0.1967. Most of the bulk chondrites analyzed have ^(143)Nd/^(144)Nd and ^(147)Sm/^(144)Nd 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 ± 0.03 AE and 4.564 ± 0.037 AE were obtained for these meteorites, respectively. The initial ε_(Nd) values obtained for Moama and ADOR are 0.03 ∓ 0.25 and 0.14 ∓ 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. ^(146)Sm-^(142)Nd systematics of ADOR and Moama are supportive of the hypothesis that ^(146)Sm was present in the early solar system and suggests a ^(146)Sm/^(144)Sm ratio of about 0.01 for the solar system ∼ 4.56 AE ago. This inferred high ^(146)Sm abundance cannot be explained as a late injection from a supernova and must be due to galactic nucleo-synthesis.

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