Sm-Nd isotopic evolution of chondrites

The ^(143)Nd/^(144)Nd and ^(147)Sm/^(144)Nd ratios have been measured in five chondrites and the Juvinas achondrite. The range in ^(143)Nd/^(144)Nd for the analyzed meteorite samples is 5.3 ε-units (0.511673–0.511944) normalized to ^(150)Nd/^(142)Nd= 0.2096. This is correlated with the variation of 4.2% in ^(147)Sm/^(144)Nd (0.1920–0.2000). Much of this spread is due to small-scale heterogeneities in the chondrites and does not appear to reflect the large-scale volumetric averages. It is shown that none of the samples deviate more than 0.5 ε-units from a 4.6-AE reference isochron and define an initial ^(143)Nd/^(144)Nd ratio at 4.6 AE of 0.505828 ± 9. Insofar as there is a range of values of ^(147)Sm/^(144)Nd there is no unique way of picking solar or average chondritic values. From these data we have selected a new set of self-consistent present-day reference values for CHUR (“chondritic uniform reservoir”) of (^(143)Nd/^(144)Nd)_(CHUR)^0 = 0.511836 and (^(147)Sm/^(144)Nd)_(CHUR)^0 = 0.1967. The new ^(147)Sm/^(144)Nd value is 1.6% higher than the previous value assigned to CHUR using the Juvinas data of Lugmair. This will cause a small but significant change in the CHUR evolution curve. Some terrestrial samples of Archean age show clear deviations from the new CHUR curve. If the CHUR curve is representative of undifferentiated mantle then it demonstrates that depleted sources were also tapped early in the Archean. Such a depleted layer may represent the early evolution of the source of present-day mid-ocean ridge basalts. There exists a variety of discrepancies with most earlier meteorite data which includes determination of all Nd isotopes and Sm/Nd ratios. These discrepancies require clarification in order to permit reliable interlaboratory comparisons. The new CHUR curve implies substantial changes in model ages for lunar rocks and thus also in the interpretation of early lunar chronology